I'm running Vista 64-Bit, with 8 GB's of ram. My ram is 800mhz DDR2, so the DDR3 stuff you have on an i7 board will be faster.
In my experience, the game almost never uses more then 5.5 to 6gb's of ram, even in cities.

I'm using a 4870 X2, for five clients on one screen 2560x1600. I've symlinked the five wow folders, which has greatly increased my performance in cities. My main window has all of the eye-candy cranked up, but the four other warcrafts have minimal settings and max fps of 8 (I believe) while they're in the background. Even with all effects up a fair ways on all windows, running around on the ground did not challenge my system. However, without reducing the effects on my background warcrafts, roughly 1 in 2 flights, one or two of the smaller windows would freeze. As far as handling the video, the card is overkill for warcraft, as is your 4850.