Assuming you have a fairly good CPU, you can probably handle any graphics card on the market. For example, a non-overclocked i5-750 would probably average 50% usage for most high-end games with an HD5870 1GB. It varies a lot, some of my games only use 20% and a few go up over 60%.
The i5-750 is probably the best overall value when building a medium to high-end system. You can probably get a $100 CPU that can keep up with an HD5870 in most games but with an i5-750 you can get another graphics card for SLI/Crossfire or a faster card in a few years and not upgrade the CPU (you may need to overclock depending on what cards you have). Your choice of CPU also depends on whether you have any other CPU intensive requirements like video encoding.
*For most game benchmarks, an i5-750 is comparable to an i7-860 or i7-920.
An i5-750 and HD5850 1GB is a really great value high-end system (and 4GB DDR3).