That mobo with its 925X chipset will only support 90nm CPU. C2D is out of the question on that board. The bast you can get is P4 EE, which is not much of a performance gain from your current CPU.
I think the best, cheapest wayy for you to gain a bit of performance is to get 2x 1GB DDR2533 RAM. I don't know if you have one stick of RAM or 2x 512 atm. If you have 1x 1GB than just add another,. If you have 2x 512 sticks then try to sell them and get 2x 1GB sticks. This will be a slight improvement, but nothing compared to a new mobo with C2D or quad.