Here is Tom's most recent GPU comparisons
at various price/performance points for gaming.
-- You have a X58 chipset, top of the line for GPU support: it can crossfire ATI or SLI Nvidia cards.
-- Tom's recommends 2 HD 4850s in crossfire at $200.
-- If you use PCI expansion cards say, for example TV and sound, then you cannot use 2 double-slot GPUs. Then get one HD 4890.
-- If yours is a gaming computer and you can afford $400, that puts you in the HD 5850 with DX11 support (if you are using Windows7). It also allows you add another 5850 when you can afford it.
The choice really comes down to your budget, how you plan to use your computer, and whether or not you want one or two GPUs.