Unless you desperately need PhysX, you should stay open to both ATi and nVidia.
In your price range (~$200), the best cards you can get are the Radeon HD 6850 (IMO the best bang-for-the-buck card out there), or the GTX 465 1gb. You should really be deciding between those two.
If you’d like to save some money, the Radeon HD 5770 is in the $120-140 range, but it doesn’t perform nearly as well as the above cards (its somewhere around a GTX 260 and Radeon HD 4870 performance wise). Its still one of the best in its price range tho, and has DX11 too.
I’d personally recommend the Radeon HD 6850, which when crossfired with another 6850 performs just as well as the 5890 in many games, including crysis. It also requires only a 6 pin connector, and draws less power than the GTX 465 (see this review for comparisons).
But… let us know what power supply you have first. If price and power draw are both serious concerns, you’ll still notice a very significant performance increase with a 5770, and if your current system handled a 8600gts it should handle a 5770 quite fine.
Google “8600gts vs 5770” and see what people are saying, like on this page.