I need to update my PC processor?

You would be better off getting a new computer at that point. The pentium 4 used a different socket 2 generations ago. Agp has been replaced with pci express 1.0 and 2.0. If I was in your shoes and you don't play very many games intel is about ready to release a new platform called sandybridge. When thats released computers with core I3 and I5 are going to drop to where its going to be real cheap to purchase them. Or if you want you can get a sandybridge computer that has its own decent gpu built in the CPU. Honestly a new computer wont hurt lol.