A Good Graphics Card for Mac Under $200?

THIS IS A MAC HARDWARE QUESTION! IF YOU LEAVE ANY RUDE MAC BASHING COMMENTS I WILL REPORT YOU. Thank you for your cooperation. I play Halo on my iMac, and it has a decent enough graphics card (ATI Radeon X1600), but the framerate gets extremely choppy at times. Does anybody know of a good graphics card (256 mb vram+) in a price range of $50-$200?