Performance wise, the countless reviews see the 8800's in front in almost all the games. Top that with a more power hungry setup for crossfire and you have very few reasons to chose the ATI solution. I'm saying this with a little tear in my eye since i only owned ATI cards ever since 2002 and was only recently "forced" to go with a nVidia card because of the unfortunate price/performance ratio of the 2900XT. I waited and waited for it and then when it was released i was underwhelmed and should've bought the 8800GTX much sooner.
Duall 8800Ultras will own the Crossfire 2900XT system, but value for money, the 2900XT kicks. The 8800Ultra system will cost near double the Crossfire. I'm actually surprised that you didn't buy the dual 8800Ultra setup when you bought your Falcon Northwest (your sig). Can we have pics?