I'm using an intel Core2Duo overclocked to about 3ghz (that is still very powerful imo), but the situation often can get pretty out of hand.
i'm usually hooking 1-3 effect to 1 VSTi, but the overflow can happen if i example run/automate even 2-3 very complex basses or strings (means hell lots of oscillators modulating each other, fast LFOs included and etc.) VSTis simultaneity.
About channels, hmm i like to spread out my work place, means that i use a separate channel for every type of sound (but that's not really matters because it will give all the midi messages to a same specified VSTi, just gives me comfort when i edit/ more precise preview)
So i use usually about 10-16max for drum, ~6 for bass, ~6-8 for strings, ~4 for leads, and you can count ~10 more for "FX" and other "filler" sounds that will make the experience more richer.
There you have it, about ~40 sounds at a time at least. (means i haven't even counted the multiplier that you need to include on polyphony sounds) I can easily reach 3-400+ :shock: sounds at a time because of their extended release time.
Talking about the need for every single extra way to free up my cpu

, i very much would like to ask for the multicore support. ^_^
I'm thinking anyway to buy a strong 3-4ghz dual/quad core processor and clock it 'till it lets me. That way i will breathe a bit more easier with polyphony. =)
complex strings and very fast LFO based basses are the most tricky ones. (cpu hogs)