If your never going to sli, then the 9800gx2 is by far your best option.. I dont know if your just preference on nvidia which is cool its a good company, but if you take a look at ati they just release there 4870x2... Still has only beta driver but even now its kicking a**... But given your choices 9800gx2 by far its not even really a match fps or benchmark wise... Out of the 9800gx2 you'll be getting better than gtx280 performance.
Phenom 9850 x4, Asus MVP Deluxe , 8gigs OCZ ddr2 1066, Visiontek 3870x2, Xfi Elite,2x WD Raptors 10k in raid 0,Antec 1000watt, and Cooler-master Stacker 830 Evolution.