Assuming you have XP, since you have a P4 processor, I would still suggest at least 2Gb of RAM. I've got a P4 with Hyper Threading, and it runs Vista Home Premium superbly on a 1680x1050 monitor and 2.5Gb of OCZ gaming RAM.

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:

This is all RAM that should fit:

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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.