EDIT: ALMOST FORGOT!!! MOST IMPORTANT!
Get Vista 64-bit!!! NOT 32-BIT! If you get 32-bit you're not going to be able to use much RAM at all. With all of that memory from the video cards, you'll be even more limited by how much RAM Vista 32bit can use. GET 64-BIT! That way you'll be able to use every last KB of memory you have on that thing
I somewhat agree with Ferarri, with that said-
DDR3-1600. You are going to notice a performance increase in games over DDR2-1066. Up to 10 frames per second most likely. That could be the difference between a game being playable and not. You could probably overclock those sticks to at or around 1800mhz, which would yield an ever nicer performance increase.
I suggest doing one of two things video card wise-
Keep the 9800GX2's and have quad sli. It'll be a straight up power house and it will eat any game you throw at it, even Crysis on High at higher resolutions, computer monitor max resolution permitting.
However, it's not near cost effective, but going by your build that's obviously not a huge concern of yours haha. The other option is to SLI 2x 8800GTX'2, 2x 9800GTX's, or Tri-SLI one of the two. It'll still be a power house, you'll get near the performance of quad sli, and it will have a much better price/performance ratio.
Besides that, great build, congradulations. Make sure to post pics of the beast