Ok I'll tackle this one at a time.
1. Don't get the GTX 285, the 5850 outperforms it soundly and is cheaper. It can be difficult to aquire one, but possible. Just be patient. Getting anything that isn't an ATI 5000 series right now makes absolutely no sense. If you "exclusively" buy nVidia, stop now.
2. You need to post what the model number and brand of that 850w PSU are.
3. Th Asus P6T supports crossfire and SLI at x16/x16 bandwidth.
Seriously, DO NOT GET THE GTX 285, it is an extremely bad move.