Punky, deffo ditch the 8800 as its not up to scratch for the more modern games coming out.
How much you thinking of spending as like Zing has said the i5's are out and about now and a mobo you can get for 85 Gigabyte P55-UD3 and the proccy at 150, only thing that you would need is memory for it and then you got a machine thats future proof
so add in a 4870 or 90 and the 4gb of mem and you'll be on around 4 notes.