What card do you have right now? How much do you want to spend ?
What resolution monitor ? For 20 inch montitor at 1650 x 1XXX
ATI has released a new gpu based on a 4890 die, (with less stream processors)
Its called 4860---- http://www.fudzilla.com/content/view/16851/34/
For 130.00 dollars seems like a very good price. A 4770 is 100-115. Both will give excellent performance. For Nvdia a gts 250 is 110.00 at Newegg right now , thats a good card at that price.
If you want to go DX 11 ATI's new cards, the 5770 is as fast as anything listed above, probably the fastest in some games ,but thats about 170.00