Well, some will depend on what your long term plans are.
ie: Do you plan to start using a camera that records in AVCHD? if you do, although you can go through a long transcode process on your G5, you'll probably want to invest in a new intel based Mac to take advantage of Final Cuts (or even the new iMovies) ability to import AVCHD footage and convert on import to ProRES or AIC. Do you want FCS3? If you do, you need an intel based Mac.
Obviously things like more ram, more storage, fast storage would help - but really it's time to look into moving into the intel Mac platform to take advantage of the latest technologies.