I agree with Dough1397. Unless you're Donald Trump, upgrading from a P4 3.6GHz to a comparable AMD64 would be as senseless as replacing your current CPU with an identical one (along with a new motherboard, I might add). Just get a shitload of RAM.
But then again, you use a 3.6GHz P4 for "download" purposes. A 15K SCSI drive is too slow? I don't think I'm going to talk to you anymore. Since you seem to like jumping around with expensive parts (like your Raptors), I see no problem with 'upgrading' to an AMD64. In fact, I think you should buy two, one on yellow and one on green PCB, so you can switch when your mind changes again. :beer: