It all comes down to performance bud.

Choose which Pentium 4 processor you can afford, and then look at which Amd 64 processor you can get for the same money. Look online for benchmarks and see which one is faster.

You dont need to worry about Hyperthreading or 64 bit at the moment, as Maya doesnt take advantage of that YET..However, I expect future versions to run under a windows 64 bit operating system...