In looking around once, I found a different hard drive quieting solution. It's based on the fact that the hard drive ends up using the case as an amplifier to its vibrations. The NoVibes product isolates the hard drive by suspending it via some rubber bands (O-rings, actually). I've never seen one of these in person, much less ``heard'' or used one, but it seems to make sense. Try removing your hard drives from the case and holding them in your hand while they power up to see if they seem quieter. If so, you might want to look at getting some of these.

Also, keep in mind that the SilentDrive enclosures are not recommended based on spindle speed, but on heat dissipation. If your drive generates 5 or fewer Watts of heat, then it should be okay inside a SilentDrive. 5400RPM is just a guideline if you're unable to find specific information on your drive.