Storage has never been cheaper. At the time of writing, just £30 secures you a hard disk drive that adds 40GB of storage to your PC, and if you're willing to spend a little more, then you can significantly boost your computer's capacity.

As ever, though, you're faced with options. We'll come to connectivity shortly, but first let's deal with the inner workings of the drives themselves. Capacity aside, the next thing you're likely to meet is the RPM, or rotations per minute. This is basically how fast the disk spindle spins, and the rule of thumb is the faster it goes, the faster your drive can read its data.

For desktop PCs, 5,400rpm is the lowest you should consider (and even that only at a push, or when buying an exceedingly large drive), while 7,200rpm tends to be the most common. 10,000rpm drives are slowly becoming more commonplace. Drives are available even faster than these, but they require a different kind of connectivity. Incidentally, laptops tend to have slower drives which consume less power and generate less heat, with the premium laptop hard drives currently spinning at 5,400rpm.

Access time is next, which is as explanatory as it sounds. It's the time taken to get data from the drive to your PC, and this is stated in measurements of milliseconds. Towards the budget end of the market, you'd expect to see drives with 8ms access times, and as you spend more you can realistically expect to bring this down to 5 or 6 - even less if you're really willing to shell out.

Then we need to consider buffer size. This is the amount of cache memory that your drive is equipped with, in which is stored data that the drive reckons the system will request next. Clearly, the bigger the buffer, the more data it can hold and thus potentially it can deliver its data quicker. Budget drives tend to have a 2MB buffer, although 8MB is common too. Opt for the latter if you can, as it can make a noticeable difference.

And so we move on to the inevitable question of capacity. Most people determine their hard drive choices by the amount of data they can hold. If you're snooping round the second hand market, you may still find drives with less than one gigabyte of storage space, but these are of little use with today's bloated software, and wouldn't even hold an installation of Windows XP.

No, it's more sensible to set an absolute minimum of 20GB, and then to buy as much storage as you can realistically afford. For a standard family PC, 80 to 100GB should suffice, while for gamers or those involved in multimedia work, 160GB is a decent choice. However, it may actually work out better to buy two drives with 60-80GB capacity each - check out the information on RAID lower down this guide.

Now we've discussed the inner bits of the drives, we need to focus on how they hook up to your system. We'll start with drives designed to be seated inside your system box. Most drives available, particularly in the budget sector, tend to be either ATA100 or ATA133. The 'ATA' bit stands for Advanced Technology Attachment, and the number relates to the speed it can shift data around - either 100 or 133MB per second.

Some motherboards limit support to ATA100, although ATA133 drives are backwards compatible. Drives that use this standard need quite wide cables, and that can have an impact on airflow in a PC case. To the majority, that won't be a big deal, though.

To those it does bother, then Serial ATA has recently arrived. This is currently the format of choice for power hungry PC users, although the fact that it's in its infancy inevitably means some users will meet teething problems. Basically, Serial ATA - or SATA - can, when properly optimised, move data at up to 1.5GB per second.

A motherboard needs to be SATA compatible for maximum impact, although note that some boards have cheated. This means that while they accept a SATA drive, they channel its data through existing technology rather than supporting it natively, meaning you don't get the benefit of its potential extra bandwidth. What you do get the benefit of, though, is the thin cable required to connect a SATA drive up to a motherboard. That's far friendlier for airflow inside the case. Expect the overwhelming majority of desktop PCs to include SATA as standard within the next eighteen months.

One thing worth keeping an eye out for is RAID, which opens up some interesting options if you have more than one drive. RAID stands for Redundant Array Of Independent (or Inexpensive) Disks, and it allows you to group drives together in one of two ways.

The first allows you to group (or 'stripe') together all of your hard disks as one, so your PC sees them as a single entity. This is known as RAID 0. The advantage here is speed. Your machine will take turns addressing each drive, in order to maximise the data cache on each. You can get some healthy performance gains, but the risk is that if one drive goes down, they all go down. That's because no full file is written to any of the drives, instead they each house fragments of files. Still, it's an economical way of getting, for instance, 160GB of storage, as using two 80GB drives will probably be cheaper and faster.

RAID 1, meanwhile, is a backup tool, as each drive becomes a mirror. This slows things down a little, as each file has to be written to each drive, but if one goes down, the other can quickly step into its place.

There are other versions of RAID, such as RAID 5 (which is basically RAID 0 but with the ability to get things going again should a drive fail) and RAID 10 (which mixes in the performance boost of RAID 0 and the backup facility of RAID 1, but requires four drives), but for home use it's likely to be RAID 0 or RAID 1 that you're interested in. Either way, your motherboard should have RAID support if you want to consider this.

While we're talking about technologies that may not appeal to the home user, we should briefly mention SCSI connectivity. SCSI (Small Computer Systems Interface) is currently the fastest way of getting data between a computer and a hard drive, but you generally have to pay through the nose for it. Note that technologies such as SATA are closing the performance gap fast, too. You'll find SCSI generally in professional environments, as it tends to be employed by the likes of professional video editors and people making the next Toy Story wannabe.

Let's round things up with a look at external options. An external hard drive is generally the same beast that sits inside your machine, but in a nice case with extra connections on it and a higher price tag. In fact, it's often more economical to take an existing drive and buy a special housing unit for it if you want an external drive, as a complete off-the-shelf solution rarely seems to be a bargain.

Being external, the drive has a tougher job getting data into your machine, and thus the price you pay in performance terms is that data transfers are notably slower. Nonetheless, you do gain an effective storage solution that won't go down if the rest of the inner workings of your PC do.

You should be looking for similar specs with an external drive as those described above, with one additional concern. That's the way that the drive hooks up to your PC. FireWire, or IEEE1394, is the fastest method (although it requires your PC to have a FireWire interface on it somewhere), but USB 2.0 will be fast enough for most. Avoid connecting via USB 1.1 if you can, though, as data transfer will be painfully slow.

Also worth a brief mention are other types of drive, such as USB pen drives (also known as flash drives, key drives, etc.). These again work best on the USB 2.0 interface, and are available in capacities of a gigabyte or more. Some make use of actual miniature hard drives, but most use flash memory that is a bit slower, but has no moving parts. Look for additional features such as built-in encryption that can help prevent your personal data falling into the wrong hands.

Hopefully all of that has given you food for thought. It should be said that, while we've outlined many options to you here, few of them will make a huge difference to your PC. Nonetheless, don't ignore them, and try to go down the routes that will give you a boost, such as SATA.