Ha, funny thing, im in the same exact boat. I too have a Pentium 4 3.2 HT, 2 gigs RAM, and a nVidia 7800GT in Windows 7 RC1, and back on XP i got 120 on the stress test, and with the EXACT same settings in Windows 7 i get ~90 FPS, and in game I get random frame drops too, mainly when looking at a far distance.
I think its crazy how i have lost so much framerate by going to this Windows. Its my only complaint though, Windows 7 is 30x better than any other OS out there, its just kinda stressing. I hope they solve this frame drops.
Frame drops in TF2 and CSS. Trying COD4 tomorrow and will see what happens.
Also im getting some older Omega drivers
for Vista64 and will be trying them out tonight.