well, a top of the line pc cost much more than a G5. Getting a fast CPU is only one part of the machine, it will slow somewhere else.

If you plan to go strong on Maya, I would advice you to invest in RAM. It will give you the biggest bang for the buck. Other strategy would be to go for fast HD. Get a 36gig Raptor now and latter get another one and put them in RAID. If your work is diskbound, it can imprve performance by 10-20%. And best of all, when you get your new machine, you can transfer the 2HD in the new oneand get back your investment. Finaly, other possibility is to get a pro videocard. But this will only help you if you have a crap load of poly on the screen or need preview of the material, etc... It wont help on render time.

As you can see, I prefer modular upgrade, especialy the ones you can transfert from one pc to the next since its a nice money saver. Dont think that buying a sub 1000$ system would be a great investment, they are only good for gamers or to surf the net. High end machine cost much much more.