Hey J-Mac,
ok gotcha now....
well the main differences between your machine & the MDD (mirrored drive door) machines outside of CPU speed are:
1) DDR RAM (which unfortunately the Moto/G4 chip cannot really take full advantage of, but probably helps somewhat)
2) room for 4 HDDs & addition of ATA/100 Bus. main HDD bus is ATA/100, secondary (that is used for optical drives is ATA/66)
3) System bus is 167 MHz. which IS 25% faster than tour @ 133MHz but not blazing either.
4) maybe a better video card depending on wha model u get.
IMHO, basically what it comes down to is this...
u have to do some homework. make sure that the MDD machine u would upgrade to has L3 Cache (ideally 2MB/chip). i dont know if ALL models have it. sometimes apple will not include it on the low end model. (it wasnt on the original Quicksilver 733 machines) L3 greatlly improves the efficiency floating point processing (which is how just about all DAWs process audio w/ the exception of PT). the Gigadesigns upgrade cards have 2MB of L3 cache per chip, but running @ only 1/5 the processor speed. this might be better using Apple supplied dual CPUs.
if u don't mind the hassle of switching computers then the \"newer\" G4 is probably better than an upgrade, if just for the faster System & ATA busses. but as Ghoost said, in OS9 they quite loud as there is no intelligent heat/fan management. There is in OS X, & they run considerably Quieter.
an upgrade to a G5 will never be cheap, no matter how u cut it.
personally i dont think there is a single model thaty comes w/ enough RAM or HD space (i have 240 GB in my G4, & most of that space is occupied) so in addition to the purchase of the G5 u will need to stock up on RAM (OSX is RAM hungry & from what i understand if u load it up w/ 4GB or more u get a SEVERE performance boost as OSX no longer needs to use swapfiles) anyway...2GB of RAM isn't cheap, nevermind 4 or 8GB. plus u will want/need a secondary HD. SATA drives arent particularly expensive (unless u buy the 10kRPM ones) but u still have to pay for them...it all adds up.
i've been a mac user going on over 12 years, & have done the major model upgrade dance a number of times. if i've learned anything its this...let the dust settle...its almost has. UA has worked out most (but not all) of the kinks using a UAD in a G5 system. i would let them get it all sorted out before i spent major $$$, thats why i bought the Gigadesigns upgrade. its a stop gap until the G5/UAD-1 compatibility issues are completely worked out (not to mention whatever issues exist whithin the DAWs). my G4 dual 1.33 +2 UAD setup will probably give me another year to 18 months of good use (Unless logic 7 is a hog) before i start feeling the need to upgrade again. by then dual 3, 3.5, & maybe even 4GHz machines will be readily available (or 2 & 2.5GHz models used for cheap).
hope all my chatter helps,
-dave