An Athlon 64 3000+ isn't too bad. The only thing that won't be really good at is intensive HD work.
You don't want a G4 at this point. I expect that all of the future Mac software you'd want to run will probably be Intel-only in a year or two. Apple offers academic pricing, but I think the savings is maybe $50-100--not exactly a jaw-dropper, but they do sweeten it a bit.
Consider this, in addition: most of the big software developers also offer academic pricing. Some of those licenses are a really great deal. (For examples, check out the following company who came to the top of my head who does not advertise with us and does not constitute a comprehensive listing of companies selling educational software licenses:
http://www.journeyed.com/home.asp .)