Ah okay, so even with the fastest clocks available today, the best you could hope for is cutting the 8 hours to 4, or one month to two weeks.
As far as the parallel cores it sounds like you already have thought about possible "end runs" around MySQL's "transaction safe" ALTER (which is probably why it's single threaded). InnoDB supposedly has a few performance improvements for ALTER over MyISAM so you are already taking advantage of that.
Intel(R) Xeon(R) CPU X5570 @ 2.93GHz
So it was a pretty high-end Nehalem proc already. And, yeah, we do have ECC RAM in the box, of course. Plus InnoDB does page-level checksumming to guard against corruption too.
But still, it was ~8 hours or so. Using all 8 cores on the box would have brought it down to ~1 hour if it could have been perfectly parallelized (and much of it could have been, I bet).