AMD64 is a great processor, however, I would not run the AMD64 unless you will be running windows 64bit version or a linux 64bit distro. The chip is fast but it will still only use 8 registers (not the 16 it has) when it is running on a 32 bit platform like windows xp home and pro.
If you want an AMD64 the way to go would be to get a dual core AMD64 which will act just like dual processors (still running in 32bit each)... but running a single core AMD64 will not give you better performance then a comparable Intel processor.
You might be able to find an AMD64 cheaper than a comparable Intel but dont buy it because it will give you better performance on windows xp because it won't.
