While it's still speculation at this point whether AMD's dual core solution or Intel's dual core solution will be better, if the CPUs are on fairly "level footing," it'll likely be the Intel dual core, as the P4s are simply much better at encoding and the like. This is coming from an AMD fan/anti-Intel guy but if you're building a strictly video editing system, I'd go with a P4.
I saw that someone asked earlier about the memory...generally, Athlon 64s like lower latencies better while P4s like higher speed better. I wouldn't worry so much about the latencies in the RAM you buy as long as it isn't nightmare-inducing or anything.