You aren't specific enough. Do you have a budget?
You can get great value out of an HD6870 which can run some games at their maximum settings (i.e. Bioshock), other games on HIGH and some really demanding ones on MEDIUM.http://www.tweaktown.com/news/23425/deal_of_the_day_xfx_radeon_hd_6870_for_129_99_after_rebate_shipped_free/index.html
If you want the best single card I recommend the GTX680 which is $500.
It also matters what CPU you have as a slow CPU will bottleneck the system anyway.
- best VALUE is HD6870
- best single GPU card is GTX680