If you're a single GPU guy get a GTX 280. If you want SLi I would think two overclocked GTX 260s would be enough for anything. The latest cards are really going to shine 1920x and up with AA piled on. I've used multi-GPU, SLi and Crossfire. Even when my GTX has put up lower numbers than the other methods metioned the FPS has been much smoother. I'd expect the GTX 280 to be even better. With the other methods I'd get odd glitches, hitching and wild FPS fluctuations. Also at some point 256Mb interface cards with 512Mb memory will run out.
I'd prefer SLi over Crossfire because nVidia have more than one method of AFR. When standard AFR doesn't work in a title with Crossfire you are left with worse than a single card performance. At that point you may disable it to get back to single level of performance.