A 5400RPM drive will be just fine if you aren't doing high end commercial stuff. All of the drives that come pre-installed Macs are 5400 RPM. My wife does quite a bit of iMovie editing with her 1st, 2nd, and 3rd graders (who have very critical eyse) and haven't had any problems. In fact, she burns some of the iMovies to DV, and some back to tapes for parents and they love the quality and can't believe it was done by the class and not professionally....
so you should be fine with a 5400 RPM drive.
good luck
dave
btw, we have a maxtor 80GB, Lacie 40 GB, and Western Digital 30GB, and haven't had problems with any (we like the Maxtor the best because of the sheer size)