If you are intent on Nvidia, and willing to drop some good cash, waiting for the DX11 cards is not a bad idea. Sli 8800GTs is still a very potent setup, and spending money on a 285 might be a mostly sideways upgrade at 1920x1080, and may not give you the satisfaction you are looking for in an upgrade.
well if i were you i would buy an cooler master 212+ cpu cooler and overclock the processor to at least 3.2 and then i would buy at least an 5850 graphics card from ati. that already should show some increase in performance
Compare those charts, as well as the charts to other games, and the HD5870 is a good card throughout. I understand you can't Crossfire on that board, but it just might be worth your while for an ATI card.