Looking to go dual core either AMD or Intel.
64bit, sli mobo. Graphic's card Geforce7900gt ?.
None of these are going to happen with what you have. You are unlikely to manage to get as high a spec as you expect for the money you suggest.
I think you should manage to get a 64bit AMD or a lower end dual core CPU and maybe a 7800 for the money however you wont get 2 GFX cards worth talking about.
Can you add the cash from the sale of the old PC?