On towers: I run CS2 (often 3 or more components simultaneously) on a Sawtooth with a 1gHz upgrade. So I'd say that a single processor above 800 would be fine; I ran mine as a 400mHz up until I installed CS2, which lagged too much on the old low-end processor. But yeah, a dual will definitely be faster, but you might not notice that huge a difference.
As for music, quiet is more important. There are several things you can do (again, I run our studio on a G3 b/w, now with a ZIF upgrade to G4 550, but it ran fine with the 350mHz G3, though I do use an older version of Cubase). I'd suggest the same machine as above; a couple ways to cut down on noise is to replace the fans with noise reducing fans; I've found that strategically adding a fan or two easily compensates for the difference in strength of a single one and is a lot quieter.
But there are several ways to go about it; our studio has a control room, so we don't worry too much. But the easiest way to circumvent noise is to build or buy an enclosure (something as simple as a desk with a closeable cabinet, glass door is fine, will work). You can also use sound buffering materials on the exterior of the tower, and build a "wind tunnel" (sorry to borrow your wording there, Blue Velvet) out of it; we just used a desk with a closed cabinet, lined it with egg crating (foam, not container), and built an air vent and intake out of old PVC tubing. It was simple and worked perfectly until we moved into our new space.
All of that said, my G4 1g, even with the upgrade (which has a fan built in), is still worlds quieter than the duals I've seen.
EDITED TO ADD: I don't know much about Final Cut, but Amazon lists the requirements at G4, 500mHz, so I'd say a single gig would run it just fine.
