After much comparison, research, pondering, and back-and-forths, I've decided to go with the 17" iMac over the dual 1.25 PowerMac. Among the benefits, I valued silence the most. Apple has consistently screwed the PowerMac over the last few generations with poor-quality fans and noisy power supplies.
What's even worse than poor-quality is inconsistent quality (where's ISO-certification when you need it?), which is exactly what the PowerMacs have. Some PowerMacs are obscenely loud, some have headache-inducing whines, and others, the lucky few, are relatively quite and peaceful. It's a $2,000 lottery that I'm unwilling to play.
Existing PowerMac owners can take some comfort in Apple's recent acknowledgment of the problem. But it has taken months and months to reach that state and there's no guarantee that Apple's adjustments will cure the problem.
What continues to escape me is why Apple will let the reputation of $2,000+ machines be tarnished to save a few measly bucks on proper fans. Dell solved this problem a long time ago and their computers are virtually silent and they sell for much less than the offerings from Apple.
I'm disappointed, yet eagerly awaiting my 17" iMac, which I'll hopefully be typing from Monday or Tuesday come next week.