>But could you tell me what is the best card
That depends on what games you play and in what resolutions. One card may be better in HL2 and the other in Oblivion. Check the VGA charts on http://www.tomshardware.com
for more info on the performance of those 2 cards.
>would it work fine on my CPU
Thats a tricky question. I *think* that if your AGP slot is 8X today, you will be fine with the new video cards (which are also AGP 8X). However, you *might* need a bigger power supply to feed these new cards.