Advice re second hard drive
Any hard disk - internal or external - is spinning at all times, and anything that spins at 7200 rpm (at least) makes some amount of noise.
Adding parts to a Dimension 8200
1) Are you running Windows XP? 2) You'll be looking for PC800 45 or 40ns RDRAM in a matched...
Hard disks are hardly the only source of sound coming from a computer. The cooling fans are noisy. Optical drives can make an annoying grinding sound. Either of these are likely to be louder than almost any hard disk you add to your computer. And any mechanical device can get noisier over its life, so a disk that sounds great in the beginning may not sound so great as time goes by.
It's more important to pick a hard disk for its performance and reliability. The only advantage of an external hard disk is portability.
Among Consumer Debt brands today, the lead in performance and reliability is constantly changing hands. The last time I looked - a little over a year ago - Seagate and Hitachi (which bought IBM's hard disk operation) were rated highly, and Seagate was thought of as particularly quiet. But what happens if a Seagate disk just happens to hit a particular harmonic in your computer (or its external enclosure)?