My DAW set up consists of 3 Win XP machines, each with quad cores & 4 GB ram, for a total of 12 CPU cores & 12 GB ram...one is a dedicated VSTi machine, one is a dedicated audio machine, the 3rd machine is there for any extra resources I may need, which so far I haven't actually needed, and I'm trying out ways to make use of it....one such way is using it as a dedicated guitar soft sim & guitar audio tracks machine on a project. I may end up using that 3rd machine as my 'mix machine' for the other two though.
I too use more audio tracks than needed, for extra takes, bit parts, comps, etc. Also I use midi tracks more than once to assign & double up on VSTi's for flavor....a luxury I didn't have on my Tascam 4-track
It's hard to say how many of each audio & midi tracks I use, it really varies and many audio tracks I do end up being comped into a much less number of tracks...but I'd say typically I 'end up' with somewhere between 8-12 stereo tracks/6-10 mono tracks, and about 8-12 midi tracks with VSTi's assigned to them. As said, I may have more going on initially, and I may exceed my above average number of tracks at times. That's why 2 Win XP machines really are enough for what I do, but one XP machine with my larger projects are too much, that's why I began using a dedicated VSTi machine, and a dedicated audio machine.