I have yet to build a dual core system, still using my old "super computer' I built many years ago. It is an ASUS board with AGP slot, (no pci express slot) with 4 gigs of dual channel ram, a 3.4 ghz p4 cpu with 1 mb cache. I currently am running an overclocked BFG nvidia 7800 AGP card. In its day it was the bomb. I just loaded Diablo 3 on it, and even though I supposedly do not meet minimum requirements, it plays it fairly well with settings turned down. The only thing I can possibly upgrade would be the video card. Is there an AGP card out there that is better than the 7800? I don't care if it is Radeon or NVIDIA, just looking for more "oomph."
No offense but wasting over $100 on obsolete tech is crazy in my opinion. As I explained your Pentium 4 will hold you back in any game newer than about 2007. You literally need 2 of your processors to run Diablo 3 at the minimum.
A 3Ghz Pentium 4 is roughly the same as a 1.4Ghz Core2Duo.