It all depends on how many articulations you want to have loaded in the background. It you want to load a full orchestra with 3 or 4 articulations per instrument, you will need a *lot* of memory. Don't even think loading it on one PowerMac or Mac Pro, your woodwinds and brass will fill up the 2.5 GB of ram like a snap.
It'd be cool if VI was 64bits by the next 12 or 18 months, but unfortunately there's nothing sure. When I met with Paul a few months ago, he seemed like going 64 bit wasn't a priority at the time. Maybe this has changed, but still...
Also, as I said earlier in this post, the day you can run VI in 64 bits, you'll also need your host to be 64 bit. I met with three guys from the Logic team a few months ago (I know, I meet with a lot people [:)]), anyway - they clearly stated that going 64 bit was clearly not a priority either. They already spent more than a year going from PowerPC to Intel, and in the meantime, the program basically didn't improve at all. As a result, most Logic users are pissed! It's been basically two years that nothing happened. So I doubt they are working on 64-bit right now...
Again, this was a few months ago, and this might have changed, but I don't think developing a 64bit version of Logic will take only a few months. I wouldn't be waiting for that one anytime soon. Digital Performer just got upgraded - it took them three years to come out with a new version, so again, I doubt they will have anything 64-bit soon.
Regarding "smaller" hosts (Rax, or Plogue), their ressources are very limited, and who knows if the developpers even know how to develop in 64-bit. I wouldn't be waiting on that front either :/
So, CPU power is definitely *not* the issue in this specific case. I would be interested in the amazing power of a Mac Pro if I could load enough samples to make it slow down. But that won't be the case. You'll fill up your memory way sooner than your CPU. Mac Pro or not, as of today, you will run out of ram pretty quickly if your goal is to have all the articulations loaded in the background. There's just no other way than computer farms (windows or mac).
Now... waiting for the 8core machines to come out "which might be April next year", is unfortunately speculation. Maybe they'll be out in April. Maybe in September... Who knows? Remember that the G5 was supposed to reach 3Ghz by 2004? I know this is not the same isuse, but my point is - you just never know when a new computer is going to be available.
And anyway, 8-core computers or not, realistically, by April next year, it is doubtful you will be able to use 16 GB of memory from your sequencer (Mac Pro or anything else) with Vienna Instruments.
My guess is that it's going to take at least two years before we're completely up and running (bug and trouble-free) in 64 bit on the Mac (maybe faster on Vista, I don't know about that).
In the meantime, investing the money for a setup that is reliable and expandable does not seem that bad of an idea...
Jerome