Graphics card power: How can I tell?

How can I tell if one graphics card is better than another? I know for Nvidia, they have different numbers and the higher numbers mean it's stronger, but then they came out with the GTX 295, a very low number, which is supposed to be one of the best. There's also the Cache, which is pretty much forced to be higher for more power. And the more recent tend to be more powerful. But is there something I'm still missing in the strength catagorization?

 

(1) Primary purpose of the computer
(2) Budget

 

Christer posted this site quite some time ago and it should more than do the trick for what you are asking. Compare Graphics Cards using this site.