Unfortunately it's no longer possible to get REALLY quiet 3 1/2" HDs, the last ones made were the barracuda IVs, these didn't go much past 60GB (from memory). Since then newer seagates and samsung spinpoints seem to be the quiestest, but TBH they are not in the same league as the old barracuda IVs -they have quite an annoying whine when idling which means that in an otherwise silent system they are going to stand out. I have a silenX PSU and a Zalman 7000 cooler and next to these the samsung spinpoints I got were just not acceptable.
So I suppose this leaves a couple of options: Go with a couple of the 200GB seagate (the only decent sized quiet drives around, samsung do upto 160GB atm), go with a much louder but better sized drive like a 250GB maxtor or, what I did, go with a small, silent drive in the HCPC and stick larger noisier ones somewhere less sensitive. It all depends on what your priorities are for the PC.
HTH
Owain