oh, here's my take on the amd 64 vs p4.
as far as perfomance goes, strait up the amd has the edge. it is faster, _in most cases_. it's also quite pricey, with the 2.0ghz opteron 146 at well over the price of a 2.8 p4.
i think where the p4 would shine at the moment is with the hyperthreading technology. in smp apps, this could be a benefit over amd's setup. however, in 'real world' type benchmarks, amd still seems to reign supreme (this is including intel's upcoming and overly used 'extreme edition'). i would, however, suggest an opteron series chip instead of an FX or athlon64. personal preference, and an FX is pretty much a 1xx series opteron.
i've been considering this issue for a workstation quite a bit. it's a tough decision
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the other option is to get a 2xx series opteron and a duallie board, and slap in another opteron chip down the road for a very fast smp setup. but the 2xx series chips are _quite_ expensive, even at the same speeds. i do not think the 1xx series can be modded to smp, but i suppose the jury is still out on that one.