If you are a hardcore gamer and want all graphics settings maxed out, then I would suggest something more powerful than a HD 4890. Otherwise you should be okay. See below for benchmarks notice the very low frames per second for Crysis Warhead, then again it is an exception, not the rule.http://www.xbitlabs.com/articles/video/display/bfg-gf-gtx275oc_7.html#sect0
You should look at the following review to see how the newer HD 5850 / HD 5870 compares to the HD 4890.http://www.xbitlabs.com/articles/video/display/radeon-hd5850.html
The HD 4890 sells for just under $200 if you can find one. The HD 5850 starts at $290. AMD is expected to release the HD 5830 relatively soon (whenever that is) which is supposed to bridge the gap between the HD 5770 (roughly $170 and not recommend for 2560 x 1600) and the HD 5850. Best guess on price is probably $225 - $250 based on current prices.