Not all 6800's come with a 2 slot design as you can see in that article
Both are great awesome cards, you really can't go wrong with either..... Yes, the Nvidia has PS3.0, everyone says that won't become a standard until probably 18 monthes from now, so unless you are a die hard FarCry player and don't plan on upgrading in 18 monthes, then sure ok
The 6800 is a 2 slot design, the X800 is not......
The 6800 will likely require a new PSU because it has requires 2 power connectors. Maximum PC tested the PSU requirements and found that you can run the 6800 on a high end quality PSU at say 400 watts barely, but you can't on a generic PSU. The X800 however uses less power then the 9800XT, so you can run it on the same PSU you have now
In my unfanboybiased opinion, even though the 6800 Ultra has a slight performance lead in the popular games and is future proof, the added extra cost of having to buy and install a new $100 PSU is just unneccesary on top of the $600 for the card
I would rather go with the X800XT, because I do plan on upgrading it in the future