The 8800GT (not sure if oc) in this article is compellingly better at gaming:
http://www.anandtech.com/video/showdoc.aspx?i=3151&p=6

The Asus P5K-V board that I am considering is a full ATX board that is crossfire compatible but the second board is only running at 4X. However, I'm not sure that the PSU could handle a second GPU (Antec earthwatts 500) so it's likely that I will stick with one card and upgrade possible a couple years down the line.