If you take a look at the list here, you can get a pretty good idea of what performance you can get out of a video card family.

http://www.videocardbenchmark.net/high_end_gpus.html

So what is your monitors native resolution? It will really help us estimate the performance of the card you are looking at getting.

That really undervalues dual-GPU cards like the 4870 X2 and GTX295, doesn't reflect in-game performance, and omits the HD5850 and 5870.

You're better off with charts from real games:

All except 58xx: http://www.techreport.com/articles.x/16820
58xx: http://www.techreport.com/articles.x/17618

All benchmark testing should be taken with a grain of salt.  I have only ever used these benchmark graphs to give a general idea of what performance video cards get.

Of course, but I still think real-world > synthetic, especially when synthetic says that a two card setup is only marginally faster.

if you buy a psu and a video card, be sure they will fit in the box you have. Video cards and psus are getting bigger and longer each year.