Probably the stock answer at that point is GeForce 6600GT (which is what I have, though I got it a while ago when they were going for about $175, because for some reason I buy good midrange graphics cards even though I'm not much of a PC gamer), though it's really hard to go too far wrong with any recent card in that range (any GeForce 6600, any Radeon X700, any Radeon X1300, any Radeon X1600, possibly some cut-down GeForce 6800 variants, possibly some cut-down Radeon X800 variants). Stay away from two-generations old cards (you pretty much can't find new GeForce 5xxx cards anymore, but there are still Radeon 9xxx cards around) and last-gen low-end cards (GeForce 62xx, Radeon X300, Radeon X600).