Depends on four things:
(1) Your budget ... if you've got a massive budget, go P4EE/A64FX, medium-high budget Athlon64, medium budget P4C, low budget AthlonXP
(2) How long until your next upgrade (the Athlon64 will last some time, followed by the P4E ... the top end chips will last as well)
(3) What kind of system you have now (i.e.., if you're running an etch-a-sketch, even a low piece of crap will seem fast)
(4) What kind of performance you need now and is it CPU intensive (i.e., running statistical analysis, video related tasks are very CPU intensive; gaming is more RAM/video intensive)