RAM Upgradation - 128MB SD RAM

Hello,

In my PC, currently I have 128MB SD RAM. The processor I have is P4. I want to upgrade my RAM. Can some one suggest me what are all the possible limitations in upgradation?. And what is the best RAM size I can go for. Right now I have very limited knowledge about the current mother board it is of Intel make having 3 to 4 slots available for RAM.

I appreciate any kind of help in this regard.

Regards,
Keshav.