The MacPro will not be doing the CPU load if you put it on the G5, the G5 will be doing the load.
Whatever machine you host VE Pro from, you will be loading the samples there. Therefore it will be the machine that will take the load. It will then pipe all the audio over it's ethernet port to the DAW machine. But the actual processing is done AT the machine that is hosting the samples.
Did you watch the VE3 demo? It will clear this up for you. However I re-read your post and it seems like all you want to do is sample your older libraries from that G5 using VE3 Pro.
What I would consider doing is the idea of making your G5 your Logic machine (or whatever DAW you use) and then have the MacPro be your VE3 machine since it will be the one that needs all the horsepower.
I have a macbookpro with only 2GB's of ram. I can run an entire orchestra from my Vista 64-bit VE3 machine. Since it is a power house and does all the processing, my macbookpro takes almost zero beating at all. And my 64-bit machine has lots of head room left. And I mean lots.
Maestro2be