For each Ghz the A64 can do a lot more, compared to the P4, so it doesn't need to go as fast in order to perform the same (or better), exactly how the two perform against each other depends on what you're asking them to do.
A shot-putter will never beat a sprinter at the 100m, but the sprinter will never win at shot-put, you know?
The A64s tend to excel at stuff like gaming, but the P4s are good at encoding tasks (like compressing DVD video into DivX format).
I would go with an A64, but I wouldn't pay the price premium for the FX line. Something like a 3200+ - 3400+ would be my choice at the moment, if I wasn't going to overclock it.
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