I've had several Seagates of both the .9 and the latest .10 versions. The older one is very quiet but the .10 makes an annoying clicking sound whenever it is accessed. I've had 3 of the .10 and purchased months apart and they have all had this same sound. My current one is inside an Antec 180 which is suppose to be pretty darn quiet with all its insulation and I can still clearly hear the drive. It is right next to an older .9 with is dead quiet.
Yet another vote for the WD5000AAKS. I got three of them and have no noise issues.
You should also look at WD7500AAKS. It uses a new technology called Perpendicular recording (which is not in WD5000AAKS yet). It's faster, quieter, cooler. OK, grain of salt, I haven't tried it yet, but the reviews are fantastic.