AMD's new Phenom II X4 955 Black Edition is the fastest member of the Phenom II family of quad core CPUs. The previous king of the Phenom II line-up was the X4 810 with a clock speed of 2.6GHz, however the X4 955 BE makes a massive leap to 3.2GHz.

Phenom II is very similar to the original Phenom and also the revised version that fixed the TLB problem. The fabrication process has shrunk from 65nm to 45nm and this reduction in the size of the transistors has freed up enough space to allow an increase in L2 cache from 2MB in Phenom to either 4MB or 6MB in Phenom II.

The other major change has been made to the integrated memory controller. Phenom worked with DDR2-1066 but Phenom II can also use DDR3-1333. AMD has come up with a scheme where Phenom II is supplied in Socket AM2+ form for DDR2 motherboards and also in a new Socket AM3 for DDR3 motherboards.

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Socket AM3 has one pin less than AM2+ so a new AM3 Phenom II processor will fit in either a new DDR3 motherboard or an older DDR2 model. By contrast, an AM2+ Phenom, Athlon X2 or Opteron won't fit in a new AM3 board so you should find that if a processor fits a particular motherboard, it will work with the memory supported by the motherboard.

For some reason AMD has decided to use a triple digit numbering system for Phenom II that lacks clarity. The quad cores are available in two forms called Phenom II X4 8xx and 9xx. The leading digits eight and nine indicate the amount of L2 cache, where a 9xx chip has the full 6MB and an 8xx model has 2MB disabled to leave 4MB. A Phenom II X3 7xx has one CPU core disabled that results in a triple core Phenom II.

The second and third digits in the model code relate to the clock speed of the processor, so a Phenom II X4 920 runs at 2.8GHz while the Phenom II X4 940 has a speed of 3.0GHz but there's nothing in those codes to tell you whether the CPU is AM2+ or AM3 which, in our opinion, is a bit barmy.

The combination of a Phenom II with an AMD 790 chipset is a processor that is head and shoulders better than the original Phenom, which drew too much power, required too much cooling and would barely overclock. By contrast a Phenom II is a match for a similarly priced Intel Core 2 Quad but the most desirable Socket AM3 Phenom II X4 810 only has a clock speed of 2.6GHz with the ability to overclock to 3.0GHz. An Intel Core 2 Quad Q8400 runs at 2.66GHz and should overclock to 3.3GHz without trouble, so the Core 2 wins by a narrow margin.

Core i7 is a completely different matter as the cheapest 920 model (which costs £250) will overclock from 2.66GHz to 3.9GHz which leaves Phenom II in the dust. Enter stage left the new Phenom II X4 955 which uses Socket AM3 with a clock speed of 3.2GHz and a price of £199. Until now the fastest AM3 Phenom II was the 2.6GHz X4 810, as the 2.8GHz X4 920 and 3.0GHz X4 940 use Socket AM2+, so this is a significant leap in clock speed for Socket AM3. Added to that, the new X4 955 has the full 6MB of L2 cache and it is a Black Edition processor that has an unlocked clock multiplier that should keep overclockers happy.

These features add up to an impressive processor that almost manages to take the fight to Core i7 920. We had no trouble keeping the X4 955 cool and quiet on an MSI 790GX-G65 motherboard and its performance was generally impressive. We overclocked the Phenom II X4 955 to 3.76GHz and it was indecently fast for a £199 processor, although the Core i7 920 delivers the goods at 3.9GHz. It's not just a matter of raw clock speed, as Core i7 supports Hyper Threading so there are eight virtual CPU cores compared to the four physical cores in Phenom II.

Added to that, the triple channel memory controller in Core i7 has 50 percent more bandwidth than the dual channel controller sported by Phenom II. The final factor is that Core i7 supports the SSE4.2 instruction set whereas Phenom II supports SSE4A, so if you are running software that uses the latest version of SSE or which benefits from threading, you'll find that Core i7 has a distinct advantage over Phenom II.

Having said that, you can buy the Phenom II X4 955 for under £200 and the MSI motherboard we mentioned sells for £100, but you would have to spend £450 on the Core i7 and a suitable motherboard. That's a significant difference of £150 and it plays strongly in the favour of the new Phenom II.

The first models of Phenom II were fairly competitive against Core 2 but the new Phenom II X4 955 is so much faster that it can do battle against the mighty Core i7. It loses by a fair margin but still represents very good value for money.