If you're talking about editing uncompressed or close-to-uncompressed HD, I'd start with at least a 1.2ghz G4.
It's not so much the processor though...the biggest (and most expensive) piece of the puzzle is a drive array that can keep up with the insane data-rates you'll need.
It's going to be 3-4-5 years before desktop HD reaches the ubiquity that DV has attained. By then we'll all be running G6s with 8gbs of ram.