Tear out the innards of this rig, and place a new one in there. (Unless you want a new box too)
C2d is silly cheap, and the prices are slowly falling all the time, with the more efficient stuff appearing, the 65nm architecture, the 'go' stepping, the higher fsb's. If you can survive on one more Gb of ram until after xmas, you'll probablty be able to make a machine that shits on stuff for 300-400 quid.
In fact
This ebuyer cart shows it all really. It is objective though, the ram that stoat got was cheaper, because overclockers had a dealio on that made some good stuff v cheap, but it is a rough guide to a very well priced and very good rig. I haven't put a gpu in there, but if you want gtx, then ebuyer have a stock of oem reference cards, these cards were made for nvidia by asus, and it's
a cupcake card for very little amounts.