Assumedly, you'll have DDR RAM because online you have a P4 processor, and assuming you have a DESKTOP (this will not be correct for a laptop):
This is the RAM I have in my p4 desktop:
http://www.newegg.com/Product/Product.as...
This is all RAM that should fit:

http://www.newegg.com/Product/ProductLis...
The more you spend on RAM, typically the faster it is. Also, matched pairs (2 sticks with the same amount of RAM in each stick) are *TYPICALLY* better than not.
https://answers.yahoo.com/question/index?qid=20080701232215AAOZwOe