George, you can use the EXS version inside Logic on a Mac with Pro Tools HD or MIX. You need Emagic's ESB, which gives you eight outputs from the Logic native engine into the TDM mixer. It works really well - I do that all the time.
But I also have to advise you to consider - consider, not necessarily decide upon - buying a Windows machine and running the VSL in GigaStudio (or possibly Kontakt later on). One computer isn't really enough, and Windows machines are much cheaper. Even if it turns out that a G5 can run most of the VSL on its own (because you can load them with up to 8GB of memory), I'm not sure it'll be practical to do everything in one machine. It really depends on what else you're doing with the Mac, i.e. how much loading and unloading you're going to have to do.
My strategy is to use Windows machines as sound modules and use the Mac for extra things within the sequencer - soft instruments, samples, etc. - that come up for each project.