Also, I'm not sure if Molex does this when you buy direct, but I've found another retailer that ships SilentDrives with a little temperature strip. You stick the strip on your drive, put it in the enclosure, and run it for a while... then the strip shows you the highest temperature that your drive reached while it was enclosed. If it's over the recommended spec, I believe you can send the SilentDrive back for a refund.
I know people who swear by them. I haven't had a chance to buy one for myself yet, though.
I have a couple of WD 7200RPM drives, and they're very quiet. Not silent, though, so I do plan to enclose them when I can afford to.
You might want to check this retailer out:
http://www.sharkacomputers.com/
I purchased my CPU and video card coolers from them, and found them to be a very good retailer. And they also sell the fluid-bearing Seagate drives. Right now, the 40GB model is on sale for $103 - a darn good price! The 80GB model is just a shade more, at $160 (but a drive that large might be a bit too warm for an enclosure).
Also, some hard drives (Such as those by IBM) have "acoustic management" features, that when turned on, cut the noise produced by the drive. Cutting noise will also cut performance some, but maybe not enough for it to matter when recording.