Paul -- I have a G5 Quad, 8 GB ram. Seems crazy that with such fast computers we're only able to limited VI counts with Vienna. I still have to do most of my Vienna tracks with my Kontakt Opus versions because of this. With the new Kontakt version 2.1.1, I'm able to get even more tracks than I could before, further widening the gap between its performance and that of VSI. I like to do most of my work with a single Mac, and am tired of the 3 G5 method I used to use. I do still have 2 G5s, but I'm hoping Vienna optimizes their VI soon, so that we can get many more instances on a single Mac. I hope Vienna isn't relying on users putting various parts of the library on separate computers to get high track counts, and in the process putting any work on their part optimizing VSI's performance on the back burner.
(I run Altiverb on the same computer, a single instance for everything.)