@ hang: The GTS 250 is generally faster than the 450 so that's not an upgrade
Anyway, the Radeon 5770 is only a few bucks more than the 5750 so that's a better choice. The 5750 is only faster than a GTS 250 some of the time. The 5770 is faster, but it's not a significant upgrade over a 250. You should get at least a Radeon 6850 or GTX 460 if you want a meaningful upgrade. Also, your CPU is more of a limiting factor at it's stock speed so you may want to consider overclocking it to get the most out of any new card.