your cooling is fine.
1) dump win98 in favor of win2k w/ SP2, or winxp, or a good linux distro.
2) update all your drivers to the latest manufacturer ones (the nvidia detonator/references ones will be fine tho - probably better than the manufacturer's even actually).
3) don't overclock anything, including your vid card.
4) u might wanna try repartitioning/formatting and cleanly installing your OS and apps from the scratch again.
5) check for viruses
all your hardware compoennts seem to be of good quality, so i doubt there's anythign wrong there..but u might have flakey memory or a PSU...also, make sure your BIOS settings are set to normal...optimal settings might be too stressful.