You will need to find out what your motherboard can take. If you still have the motherboard manual, you can find out there. otherwise, find out the name/Model/make etc either by when the PC boots up, you can jot down the name or download Aida32 click here . Its an audit program which will give you details of all the components of your system.
POst back the name and make of your motherboard and someone should ba able to help. Or you can go straight to the motherboard website and find out there.