I have an older Nvidia 64MB card that i want to replace with 256MB 68oo Nividia card. My current card is a APG4x. Can I update to a APG 8x card. I run 512 RAM with a 1.7 Pentium 4 chip. THanks much.

Almost certainly. Even if your motherboard only supports AGP4x then the card should still work, except obviously ony using 4xAGP. Look up your motherboard details on the manufacturers website (have a look inside the pc for it's make and model no.)- AGP4x/8x data should be listed.