Dell power supplies are sh*t and proprietary. As far as I konw you can't replace them without a little modding, or the new PSU won't fit right or something. I wouldn't be surprised if he was short on power... but chances are it'll work ok.
As for what card... wait and see what new drivers and patches and the enabling of features brings. However... since your x800 pro is not moddable and is lacking 4 pipes... the 6800 would probably be the better choice. At the top though, I would choose the x800 xt pe:)
Me: are you saying I can't provide?
Me: cause I know I can provide.
Me: oh and I can provide money too;)
Rachel:): why do we need money when we can just stay in our room and have sex all day?