I know that AMD chips are faster at folding than the P4, but I don't know what kind of difference HT makes. So a 3000XP is faster than a 3Ghz P4 at folding (at least in my experience).
Also the G5 is meant to be the fastest out of the Athlon XP or P4 when the folding apps have been optimised for the G5. However the new Athlon 64 may be even faster, it will be launched in Sept