Well, until you have some historical or sentimental reasons, I'd recommend against using a P4 machine. Intel kinda flopped on P4 processors generation. The only thing that P4 processors are good, is turning electricity into heat, with poor performance and huge consumption. Really!
If you need just a cheap PC, get something based on Core Duo or later. May be free as well today. These CPUs are much much better. Also extra memory will be easier to find.