i would go with the Intel, because even the lowest 64-bit AMD is still more than the 2.8GHz P4C, as is the Athlon XP 3000 @400 MHz FSB (which is 1/2 of the intel mind you, so you cant run your ram in dual channel) you gotta remember it isnt just the processor speed that makes it fast, its also the speed at which it can communicate with other devices
and btw the extreme version is so much more expensive because it has 2Mb of L2 cache instead of the standard 512Kb