I have 2 gts250 512s in my old computer. It still runs fine, it can run most any game at medium-ish settings. The big problem my old computer has is the q6600 can't run PS2. It will run blackops2 medium, BF3 low, and MW3 high, ME3 high, no problem.
If your carefull and you have some time, you can get a good GTS250 on ebay for around $30.
What CPU and board do you have? If your still running an old board like an nvidia 650i or 750i, you might be better served to save your money for a new cpu board and ram.