1. Altivec and SSE aren't really comparable. Not much is between CISC and RISC.
2. Final Cut will be quite slow and/or unusable unless you have G4 1Ghz or faster and at least 512MB RAM.
3. Beware of the Blue and White's quirks. G4 upgrades are notoriously flaky with this unit because of the firmware updating procedure.
4. Other factors like HDD performance (you'll want to upgrade HDD controller) and video card performance will give you headaches if you try to run heavy apps on this mac.
5. In the end, the list of upgrades in ram, cpu, HDD and video almost amount to a new mac mini.
Just a thought.