Upgrading my processor and need help knowing which ones will work in my system?

Since it has an old motherboard with a Pentium 4 the sensible thing to do would be to buy whatever processor you want, and a motherboard that matches. You will of course most likely need to buy new memory too. The majority of motherboards that were sold with Pentium 4 processors are too old to accept dual core processors, particularly the Pentium Dual which uses a 45nm Yorkfield core.