Intel CPUs, yes. You can choose a couple of different boards that will take DDR3 - 790i, some X48, some P45.
Why get a board that supports DDR3? Why not build one with DDR2? If you are "future proofing," then maybe wait a few more weeks for Nehalem. Right now DDR3 prices are outrageous for the performance, especially with DDR2 so cheap right now.
Or you could buy a HD 4870 for $300 and leave your MB, RAM and CPU alone because they're just fine. You'd get 45 fps at 1920x1200, again in Crysis with medium settings. I'm guessing around 50fps at 1680x1050. http://www.legitreviews.com/article/734/11/
Edit: that's because your system is massively bottlenecked in games by the 8600GT. Any money spent elsewhere is wasted, it will just make the bottleneck more spectacular.