option 1: palit 8600gt 512mb gddr3 fr 3.5k if u intend to play with current monitor and 2hrs of gaming. get a cm460w fr 2.4k total damage 6k

option 2: 9600gt/4670 + 19"LCD + cm460 . if u willing to spend on them. Any GPU above this would be bottlenecked by the cpu imo..

option 3: complete upgrade

must : sell of the 2 x 80gb and get a new 500gb hdd for 3k odd. Your 2 old hdd's should fetch u atleast 1.25k.