The Maxtor Diamond Max Plus 8 drives are VERY quiet. They are at the bottom of every performance list (well, within 3 or 4th of bottom), and while they are 7200rpm they're extremely quiet. I used to think that my Western Digital 60GB drive which I could still hear a little was quiet (5400rpm) but the Maxtor ones are even quieter!
I currently am using the 6E020L0 or whatever that model number is and its very quiet. I don't hear it at all. Its quieter than my brother Seagate U Series which can with his old computer which is a 5400 rpm. The Maxtor also has liquid bearings which is another reason why its quiet. Again, not much performance which you seem to be fine with, but VERY quiet.