Depends what effect your budget has on your choice?
The X800Pro is the least powerful with 12 pixel piplines.
The 6800 GT is more powerful than the X800Pro ofering 16 Pixel Pipelines and power not far off the top two:
The 6800 Ultra and X800XT are the two best cards out at the moment and both are out of this world if money is no object.
The N'Vida cards have support for SmartShader3.0 the next version that will be seen in new games something alot of people were surprised ATI left out.
For me if money was no object i'd go for the 6800 Ultra but other would go for the ATI X800XT the whole ATI N'Vida debate just like Intel and AMD.
The 6800 GT offers the best Value for money and can be clocked to Ultra Speeds and out Performs the X800Pro and is not to far behind the top Two,
As for the X8000Pro which is an amazing card by all standards only really consider it as the cheapest option if money is tight!!
I have a 6800 GT in My AMD64 3200 1GB PC3200 set up and enjoy 11000+ in 3DMark03 and play Farcry and Doom3 on highestsettings everything on with loads to spare so any choice with that monster PC you are buying will be fine.