Could be that you didn't uninstall all the drivers for your old Radeon card. I bet if you reinstalled Windows with only the Geforce drivers everything would work just fine.

Specs:
PIII 1 gig
Hercules Geforce 2 Pro
ASUS CUSL2-C Motherboard
256 meg Mushkin Rev 2 memory
45 Gig IBM ATA100 Hard Drive
Philips Acoustic Edge 5.1 Sound Card
Klipsch ProMedia 4.1 Speakers

Sounds like you have the heat under control so I don't think thats your problem. Are you overclocking? If so try to step it down a bit.