I have no idea, but:
"How fast is a dual 450 mhz Mac compared to a single prosc 1600+ Athlon XP windows box (just the processor power)"
very fast. Ask yourself what you plan to do with it, you want it to be a renderfarm for Maya? prolly not (and a waste of a good Mac), it will suit your needs for everything else tho. Plus you can upgrade it in the future.
"If it can be upgraded to atleast dual 1ghz l3 cache in the future. "
Likely yess, Sonnet and many other upgrade card makers now supply dual 800, 900, and 1gig modles for a variety of G4's. Look on the web for more info.