You should get an ssd. They don't have any mechanical moving parts, so they don't make any sound. They also produce less heat which allows you to turn down the speed of your casefans. The OCZ Agility is affordable and very fast. You could get a dampening enclosure for your hard disk and a dampening kit for your case, but even then you'll probably still hear your hdd. So it's better to just get an ssd.
I also suggest getting fanless graphic cards, a passive cpu cooler and low rpm casefans.
I have 2 MSI Radeon 4350 cards, both passively cooled. I have the Scythe Ninja 2 cpu cooler in fanless mode. And I have 2 120mm Scythe Gentle Typhoons spinning at 500rpm.
I also suggest getting fanless graphic cards, a passive cpu cooler and low rpm casefans.
I have 2 MSI Radeon 4350 cards, both passively cooled. I have the Scythe Ninja 2 cpu cooler in fanless mode. And I have 2 120mm Scythe Gentle Typhoons spinning at 500rpm.