(case your still alive) Check this from a previous similar question:
"The Dell Dimension 9100/XPS400 does have the i945P chipset. Unfortunately, the best processor you can get in on of the 9xx series Pentium D. That chipset does NOT support any of the Core 2 processors. I know....I had one of those machines!
You can upgrade the memory, and a Geforce 7900 GT or GS would definately improve your system performance over the X600. Considering you can pick one of those up on Ebay for $25, or buy a new one for ~$60, it might be worth picking one up.
Otherwise, save your money and buy/build a more upgradeable machine."Linky here x)