It depends on what programs and versions you're running. I use an older version of nuendo and wavelab and a load of VSTs for recording, editing and mastering, and a P4 does the job, dual core will do you as far as I'm aware as Nuendo doesnt take advantage of hyperthreading (core i processors). To be honest a core 2 duo or core 2 quad will do ya. The fastest one I've got is a Quad Q6600 and it's absolutely fantastic, I can edit video and audio and render as fast as you like. Newer versions of Photoshop can take advantage of hyperthreading so they do perform better, but for audio recording, production and mastering, I dont know any software that will give you any better performance really for having more cores.