You will end up having to buy a new mobo to support the P4, since there ARE no 700mhz P4s. If you plan on multitasking, then the 3.0ghz Pentium 4 is a good choice, but if you want to game, then you should probably consider the Athlon 64 family.
EDIT: Oh, and your sig is too big. Max is 468x120.