I understand why you went with xeons so you could use the mac base...but multiple video cards for running a single 1080p monitor/tv is complete overkill. A single 6870 or gtx470 will run every current game at max settings on 1900x1080 without a problem. The only reason for xfire or sli is with a higher resolution or multi-monitor.
Posting for help with buying computer parts after already choosing everything and spending the money makes it look like you're bragging. To most savvy computer people on this forum price/performance matters way more than who can spend the most money. That is why you are getting such a poor reception with your post. Oh, and it would help to post in the correct forum:
General Hardware