How many of us quit all other applications while we're editing video (or doing some other processor intensive task?) it probably makes sense, but I just don't do it...
Macworld takes a peek at App Tamer, an application that halts any application when it's switched to the background (I wish it would do that for individual Safari Windows!) Reviewer Jeff Porten says he's spent about an hour with it, and so far it seems to do what it claims; and it only costs $15.
Could be worth a look.
Macworld: App Tamer halts your apps to speed up your Mac