gmc2, according to the system specs
your best bet is to just buy a new system rather than trying to upgrade to play the games you want.
The tech has quadrupled since your system. everything your running is 2 technology jumps behind.
even though there are a few cards with newer GPUs like the 9400 that Sole Soul linked, PCI interface is just not fast enough to utilize that GPU, the bandwidth just isn't there. a card like this is just a waste of money. Yes it may improve upon what you have a little but it is not going to solve your problem.
todays game require more than just a better faster video card, you have ot consider the RAM and CPU also. according to the system specs you only have 512mb RAM (unless you have upgraded it) that is going to hold you back too, as well as even though you CPU is a 3.2Ghz, a newer 2.6 will still be faster in data processing because of the processing technology/newer architecture.
all in all a new video card, more RAM and a new socket 775 800Mhz FSB CPU such as one of the e series here will help you out a little but you would have to upgrade all 3 to see much difference/play without crashing. These upgrades would probably get you Oblivion and game of the same era, but you will hit a wall with game put out in the last couple years and have the same problems. all after spending about $200 in upgrades.
If you buy a new motherboard as Brickhead suggested (not a bad idea) but you will still have to upgrade your CPU, RAM and Video.
as well it looks like that system only has a 300W power supply, which means it too would have to be upgraded if you buy a new motherboard, that power supply just isn't big enough for newer motherboards, cpus etc....now your looking at $300+ in upgrades. you can get a new system for about twice that.
save your money until you can afford a new system. You'll be much better off putting that $$$$ toward a new system than upgrades. Sorry M8 but you just can't upgrade that system enough to be worth it without completely replacing everything.