The wording of your post is a little unclear, but for performance purposes, the first thing to do is to make sure you've got enough RAM that apps can stay cached in RAM. With RAM as inexpensive as it is nowdays, hey.
Load up.
I've switched from single-core to dual-core (Athlon64 to Athlon64 X2, same system otherwise) and the primary area where it helped was system responsiveness during ongoing heavy CPU work (namely, video editing jobs). With single-core, it was lethargic. Dual-core cleared that right up, as well as chopping the transcoding times in half in Premiere Elements.
As for disk, I doubt you'd notice much real-world improvement for the scenario you seem to be describing, and that's coming from a 15000rpm SCSI addict :nana: So... lots of RAM, dual-core. And if you're using onboard video, adding an inexpensive add-in video card may help with GUI performance, too.