Never knew 17,179,869,184GiB of RAM was too limiting for 10.4.x.
If you browse the VSL forum, you can read about the ram limitations. It's software related. I'm not talking about what you can physically snap into your G5. Logic itself cannot use more than 3GB of ram. So if you are using EXS in Logic with samples, you are limited to 3GB or samples, or 3-4GB in some cases. So 4GB or ram is the sweet spot. (3GB for Logic, and 1GB for OS X.)
Pro audio folks have found that more than 4GB of ram in a 10.4 machine gives a diminishing return, because of the way 10.4 handles virtual memory. So with over 4GB of ram, users have noticed that 10.4 keeps putting more and more into virtual memory, instead of using the physical ram that is on the machine. So the extra ram is NOT used by Logic, or DP, etc.
THIS is the flaw I am talking about. And it has to do with Logic being a 32bit application, as well as 10.4 not being fully 64bit.
If you browse photoshop and other graphics forums, you will see the same complaints. There are lots of gripes that "Why can't photoshop use all 16GB of ram that I just spent $$$$ on!"
So 10.5 will hopefully solve this problem, and Logic 8 will hopefully be 64bit. I don't want anything less.