I understand why you are going for 2006, the pre Core2 era is interesting, but 2006 is a rough target. It's very early in an era of gaming I tend to classify as 2005-2010, which is the late XP early Win7 gaming era of PCs catching up to and eventually surpassing the XBOX 360. Core2/Phenom and up will basically destroy pretty much anything the Socket 939 has to offer excepting the most expensive FX chips. To make your build worthwhile, I would advise you to find a decent FX processor, but good luck with that. Otherwise, your target build is pretty short lived, and in that case do not attempt to play ports from later in the XBOX 360's life cycle, as most GPU features from that console hadn't properly made its way onto the PC GPU market by then.
For the fastest videocard at the time you want the GeForce 8800 GTX or GTS, good luck finding one alive (Google Bumpgate).
If you are interested in crossfire, by all means go for dual X1950PRO as they have internal crossfire (first card to have it) and can do it without the bridge cable. The fastest GPU ATI had at the time was the X1950XTX (what that stupid model name) followed by the X1900XT and X1950PRO/X1900GT.