Many people don’t understand TCO.
The nice thing about osx is if your using shake all the render nodes are free (2k per machine on linux and you have to multithread otherwise you have to use 2 lics if you want to render per proc). Also you can get away without hiring a sysadmin which would cost you 50-100k a year(depending where you live). Managing a shop using linux without a sysadmin can be a pain in the ass especially if you have very little unix experience. Linux is not easy, especially with many machines and with inexperienced people. You are going to spend a lot of time learning and configuring linux where osx is ready out of the box. Also electric costs are much cheaper on a G5. G5 draws about 40 watts where an AMD/P4 is anywhere from 90-120. The G5 produces less heat so you will have less costs for AC.
I’m not talking out of my ass. This is comming from someone who used to be an SGI/windows/linux sysadmin 8 years ago and I know linux, tru64, solaris, windows, osx, etc… There are many situtations where I would suggest a G5 farm over a linux one. People don’t think about the cost of a sysadmin, electricity, AC, etc…
The cost difference between a cluster node G5 Xserve and a rackmount machine from Boxx is between $400-800. That cost is made up really fast when you consider all the additional hidden costs. Also buy your ram from a 3rd party. Apple way overcharges for ram and it is the same exact stuff as what goes in a pc(PC3200). Good PC3200 from mushkin is like 1/3 the cost of the stuff from Apple.
From the sound of your situtation I would definatly go with a G5 Xserve farm.