Stick with a good single core AMD proc, like that 3800+ in that link, stick with 2 gigs of memory if you can, and you don't need a 600W PSU, stick with a enermax liberty 500w for around $100.
Although, I've been running oblivion myself on my X2 3800+, 7800GT, OCZ platinum 1.5gigs, at 1024x768 with HDR, and it bogs down sometimes and has gotten choppy before.
So definetly spend the most in the video card area as you can. Step up from that 7900GS with the money you've tooken off other components.
Back on being mobile - ASUS G551- 15.3" 1080p - Core i7 4710HQ - GTX 860M - 16GB DDR3 1600 - 512GB Samsung 850 Pro - Steam ID: TheOtis