i would not saying buying the fully loaded quad g5 is rash.. I actually love mine.. but thats besides the point.
I would buy the most you can afford. Tomorrow is WWDC and I am read that the powermac replacement will be released, and I feel that if it is going to be released any time soon, then tomorrow would be the big time to do that, because all the world is watching. I would buy that if you have the money, you seam to like to keep computers for a long time, and I think that computer will last the longest into the future. Other wise I would buy a iMac core duo, and instal the ram myself because it saves money (although with the quad I just had them do it looking at the inside of that would require you to take out a few things to get access to the ram and that was not worth my time. I am only telling you this incase you decide to do the rash thing) The next option if you are still stuck on ppc would be to get a used powermac g5 low end, single processor or lower clocked dual processor, what ever you can find for a low price. Good luck getting your new computer.

post script: I would keep that computer as a itunes server if that function is still working for you, why replace something that still works? I would just expand and put that computer else where, where i can provide you your music while keeping your hard drive on your new computer empty for your songs you are going to record.