Id get the Intel Core 2 Quad Q9550, but never in my life waste the extra money for just overclockability potential on Core 2 Quad Q9650 Extreme.
(consider a AMD Phenom II 940 also, if you don't hate AMD)
9600 GT and 9800GT would be extremely underpowered for that system, so forget those.
Getting a GTX 260 (wih regular 896mb) or ATI's 4870 1GB or 4890 1GB would be the optimal solution, because you'd get great performance and still money left to do a perfect GPU upgrade in 2 years (to a then costing $200-250 GPU).







