I do most of my work on a single (aging) PC with 1.5 GB of RAM, and while I occasionally have to make creative use of Sonar's ability to flatten a track (it's a mixdown-to-wave), VI has worked great for me. The only new equipment my system has gotten in the last FOUR YEARS is a larger harddrive (the better to hold VI with).
Ignoring the fact that it's badly, badly mixed and very, very untouched up (I'm still new to the whole VI thing, and haven't had much time to experiment), I've been able to achieve things like this:
http://www.drilsej.com/BetterSound.mp3">http://www.drilsej.com/BetterSound.mp3
which is far and above better than anything I've been able to accomplish with any other library. That whole piece was essentially improvised part by part. From the first conceived note to what you here there, it was about an hour and a half of work. Obviously, not the world's greatest composition (it doesn't even have an ending), I was just doing some mixing tests.
The only tune that I've really had time to "perfect" (in quotes because it's really not perfect) is this upbeat tune I did for the end credits for a friend's film project:
http://www.drilsej.com/lastlife/6-Credits.mp3.
I think it's pretty good (if you don't have an intense hatred for synth drums), and I didn't even have to flatten any of the tracks. Not even close. That one hits about 30% CPU on my old 1.6 Ghz Athlon. Written and mixed, start to finish, in about 3 hours (which speaks volumes as to the ease of use of VI).
Anyway, I hope that helps put system performance into perspective a bit.