We need to know how much you are willing to spend on CPU/motherboard/Memory, that will tell the story right there. If the budget is somewhat unlimited, I would go with the Intel I7 920.
Actually, right now since the release of the I5, AMD is no competition. The I5 is the same price as the 965 Phenom II, the best CPU AMD has to offer and the I5 is as good if not better.
So, if you have a budget, go I5. If no budget, go with a socket 1366 I7 cpu.
If you have a low budget, like $300 or less to get CPU/mobo/ram than you'll be needing to go AMD. Yo ucan get an Athlon II quad core, decent mobo, and RAM for under $300.
But honestly, even any Dual core setup from AMD or Intel would crush the pentium 4 you have now.