I would recommend a Nvidia 8800 series card. A 8800GT 512MB starts at $189 (after mail in rebates) from what i can find on newegg.com. A little faster is the 8800GTS 512MB, that starts at $219 (after mail in rebates). I'm currently running a somewhat older version of the 8800GTS 640MB, and it runs excellent with most games. Just keep in mind that newer games aren't meant to be played at 2560x1600 at the highest settings, Crysis won't run smooth at those settings even if you throw $3000 worth of videocards at that problem, a few years from now it'll run great on low-end video cards ;-)
Also keep in mind that there are rumours flowing around that the 25th of March Nvidea will announce/release their new 9800GTX videocard. While it'll fall outside your price range, there is speculation that it might push prices of the GT and GTS even lower. You might want to wait a few days, might save you a few bucks.