I had almost the same computer you have 3 months ago (1.5 Ghz athlon, 512ddram, 40 gb hdd). I bought 6600gt in summer and 120gb HDD a month ago and everything works fine.
I had 32mb TNT2 before and i couldn't even play vice city with decent fps. Now I can play DoD source with resolution 1280x1024, full HDR and everything is set to highest expect AA that is disabled. COD2 and Aoe3 also run good.
If you have tight budget I'd suggest getting new video card, if you have money go and build yourself a killer comp. ![]()