1. Run 64bit vista (works great with WoW, other stuff... YMMV) 4gb ram and no issues with different sized monitors.
2. Your system is as good or better than 99% of the people who run 5 clients on 1 machine, you just need to set processor affinity, and max min fps
3. The main thing you need to decide is how you want your clients displayed. You could go with 2 monitors off of the 8800gtx, or 3 monitors on the gtx and pretty much any other 8series geforce card. 2 widescreen monitors in portrait mode on either side of your main screen seems to be the optimal setup for 5box 1 machine, but 1 on a 24 and 4 on another 24 is nice also. Check out the screenshot thread for ideas...