The 850W would be enough for any single GPU solution - but for SLI 580s, you might want to spring for the 1000W.
Silly question, but is the XPS case fully standard in terms of mounts? I know most OEM cases I've seen didn't like to play with standard mobos and PSUs (even if it was something small like a weird set up hookups to the front switch). If you're spending $1000, why not spend a bit more on a new case that will be much cleaner/easier/etc. Then you can sell the XPS to offset some of the cost