A useful resource I've used in the past:
http://www.overclock.net/graphics-cards-general/502403-graphics-card-ranking-5th-time-last.html
It's a ranked list of graphics cards. With so many model numbers out there, it's sometimes hard to tell which are stronger than others just by the numbers. This list can help.
For example....
GPU : Nvidia 8600GT / 7800GT / 9600GT / ATI 4670 / 5650 / 5670 / 3870
These cards range from #110 on the list (8600GT) to #57 (5670)......
GPU : Nvidia 8800GT / 9800GT / GTS 450 / GTS 250 / GTX 260 / ATI 5750 / 4850 / 4870 / 3870x2 (dual gpu chip)
These ones range from #59 (8800GT) to #39 (4870)
And I certainly hope the minimums are at least a little bit lower, since mine isn't quite as strong.
(I'm at #122, with an HD2600XT/256... was able to run Mass Effect 2, Dragon Age, and FO3/NV. On low-mid. And I can't upgrade it, since my computer's an all-in-one. Which is a shame, since I have a 4650/512 I won as a door prize. Heh.)