Personally, I would go with the new machine. Instead of just getting processors clocked twice as fast, you get ones clocked four times the speed, much more RAM capacity, 64-bit processing, Panther, the new iLife, and a lot more. That's my opinion on the matter. The way I look at it, eventually you'll want to upgrade to G5 CPUs anyway...and you might as well not regret having spent money on the G4 upgrade.
By the way, we are expecting a G5 revision soon...I would wait a few months. 3 GHz was promised for this summer....