OC the 8800GT and it dominate the 9600.
Its pretty hard to sell computers, i could never do it unless i sell it for very cheap. But then again, i dont try hard....
But even if you dont sell it by then, you might get lucky by the things going down cheaper.
It just seems like the 8800GT just keep getting cheaper and cheaper, either with rebates, promo codes, or it just get cheaper.
You could go with a cheaper PSU to help your budget, i think all you will need is a 500W, unless someone said you need more and i didnt find it in this post...
The 8800GT say that you only need 400W, and to SLI, a 450W will do fine.
You can go with the Antec Basiq
Antec Basiq BP500UB 500W ATX12V Version 2.01 Power Supply 100 - 240 V UL, CB, CE, CSA, TUV, FCC
as long as you have an extra power cord and 4 screws (or less, my friend only use 2 screws to keep the PSU in place)
and if you still cant sell your current computer, the adding extra programs is a good idea, and maybe add like another 512mb of ram to make it 1gb.