Where to begin...
I personally wouldn't bother with 32-bit Vista - that immediately limits you to 4GB. Get 64-bit and at least 4GB of memory, and since it's less than $100 extra, get 8GB. Why $220 for a monitor stand that isn't needed? The monitors will have stands. If you want stacked monitors, then use appropriate stands, but for 3 horizontal monitors, I feel that it's a waste. 750i SLI board? SLI is for using multiple cards to one monitor, and for games that do well with SLI. You're doing neither. 8800GTS.. I'm an nVidia fan, but the HD4850 is simply a faster card for less money. 600W power supply for a quad core and 2 high-power videocards.. you might want to check the power requirements. As for Q6600 vs Q9300.. the Q6600 will save you some money and perform better when overclocked, but I'd probably go with the Q9300 at stock speed and lower power consumption. Better yet, step up to the Q9450 - increased clock speed and double the cache to 12MB.

For nearly $2000, there's no reason not to go with a Q9450, 8GB memory, and Vista 64. There's still plenty of cash for the rest of the machine.