Just curious if there is any way to speed up the encoding from cd to AAC format in iTunes? What determines this speed I am currently getting on average? 6x
Your processor, CD drive, bus speed and RAM. I don't think there are any ways of speeding it up using software (unless you can find another encoder faster than itunes). If you're finding your G4 is getting a bit slow, consider a processor upgrade. I've got a Sawtooth 350 that I upgraded to 800MHz with a GigaDesigns card, it wasn't too expensive. Or wait for that 64bit PowerBook
6x is pretty good, I'd say. Why do you want to speed it up? Do you need the time or clock cycles? At 6x, an hour CD takes 10 minutes. In that time, you can do lots of stuff.
The only way to speed up something like compression or expansion is to get better hardware.
Note: There IS a way to speed up ripping. Don't let your Mac play the songs that it is currently ripping. It spends a significant amount of it's horse-power playing them. I've found that my ripping speed goes up by 1 or 2x this way.