You are running a Ati Xpress 1100 chipset or also called a 200.
The best chip it can support is the PD 360 at 3.6 ghz from my reading.
Check out this link : http://en.wikipedia.org/wiki/Pentium_D#Presler
What video card are you running since if you have the stock X300 in there it would be a great improvement if you upgraded the card since the X300 is horrible.
There are some decent PCI x 16 cards for not alot of money <75 dollars US that would really help. You would just have to watch for power requirements based on your psu.
If you went to a PD 360 and a better GPU that would help.
BUT watch the cost since if the cost of the chip and video card are more than a 150 dollars US it would be better to build a new system.
Check Ebay for the CPU and GPU just watch the seller ratings.