ORIGINAL: j boy
So I guess another way to phrase the question would be
How long until that new quad-core becomes a door-stop?
One thing I will say is that every generation gets a little bit closer to not actually becoming unusable for multitrack audio. Eight years ago, a top-line, brand new computer was inadequate the day you took it out of the box. If you were planning on running internal effects or virtual instruments or high track counts then you were going to be doing a lot of bouncing and rendering and you were going to be working at >10ms latency.
Fast-forward five years. Athlon 64 processors were right on the cusp of being able to do an entire conventional 24- or 48-track audio project in the box at <3ms latency, with only a little bit of freezing and selective use of CPU-heavy effects. High sample rates or a free and easy way with softsynths could still force you into computational compromises, though, and a dedicated effort to push the limits could certainly bring such a system to its knees easily.
I haven't upgraded to quad core yet, but I have heard rumors that performance is very close to approaching hardware, i.e. no more need to think about conserving resources or tweaking latency because there's enough reserve to do whatever you need to. If that's not really the case with this generation, I think it will be soon. And once you have a computer that does everything you need at the sample rates and latency you want, upgrading starts to become a luxury.
Cheers.