You should be able to get decent performance out of a 9800GTX, I think they have lower power requirements now than the hungry dual 6-pin PCI-E power version I had.
I can't even run Crysis 1 in Win7, fully patched the stupid thing runs for about 2 minutes and crashes to the desktop. Warhead I have no trouble with in Win 7, Crysis I have to fall back to XP to play, it's annoying.
What that is telling me is that your 450w PSU can only provide 120w on the 12v rail. That's pretty weak for a 450w PSU considering that the two components with the biggest power draw on the 12v rail are the CPU and the video card.
you could try a ATI Radeon hd 5670 it has low power requirements and no PCI connectors or a Nvidia GTS 240 Im guessing low power requirments aswell but these cards are not the best. Please what resolution do you play or try to play at?