If wanting to upgrade, I suggest looking at www.tigerdirect.com
there are alot of mobo/chip combo's that are inexpensive. upgrading through this setup is a good start. This would be a low budget way of upgrading. (you will need new ram though to match the system and if it is old IDE hard drive, make sure it has a port for the IDE hard drive).
Edit: Weigh your options though when upgrading, it might be wise to save up and get a premade system if you are not familiar with building comps. upgrading to what you want for gaming can run you +$500 easily. buying a decent laptop that can play some games is about the same price, but building the comp will cost less than buying a new comp with comparible hardware.