Gee thanx Stevo!
Anyways, the used G4 you were looking at runs at a lower clock speed that the new Mac Mini. So, since I can assume that that program could use a little more clock speed (and, hey, the Mac Mini is much cheaper), I'd still go with the Mac Mini.
Plus, the used G4 on eBay does not come with OS X, and you will need that for VPC. The Mac Mini comes bundled with it.