Fitz:
When you say that your G5 "runs out of memory," what are the symptoms?
What I'm getting at is that your G5 could be running out of CPU before it is running out memory. This depends on what OS version you are running, what application is the MIDI source and what application is hosting VE or VI. In my experience, OS 10.5.2 has more processor overhead than OS 10.4.11, Logic 7 is more efficient in terms of CPU usage than Logic 8 - - and both Logic 7 and Logic 8 are more efficient hosts than VE. Also if you are using Finale as a MIDI source to play virtual instruments hosted by Logic, DP or VE, you should be aware that Finale is a CPU hog. In my testing Finale playing a single empty staff with no notes and no MIDI data in the staff - - with scrolling & Human Playback turned off - - consumes 90-105% of one CPU during playback - - thus significantly limiting the number of virtual instruments you can employ. Your MacBook Pro most likely has more powerful CPUs than your dual G5, a new MacPro equipped with a suitable amount of RAM would probably mitigate all these issues.
In my testing on my dual 2.5GHz G5 equipped with 7GB of RAM, playing a file running under OS 10.5.2 with VE instantiated within Logic 8 produced digital artifacts (clicks and pops) with a significantly lower number of samples than playing the same file running under OS 10.4.11 with Logic 7 acting as the host for Vienna Instruments sample players.