I have bought 3 hard disks over the last couple of years and they have all turned noisy yet were very quiet to start with. When I first looked at doing a HTPC I wanted to use solid state technology but the prices where prohibitive. Recently I decided to have another look and now its much more affordable. The solution I decided to try is a 256MB DiskOnModule, this is a very small plastic covered thing that plugs directly in to the IDE connector on the motherboard and perfectly emulates a standard HD. Throughput wise these are surprisingly slow but have the advantage of extremely fast seek times. Apart from large file copies and the larger software installs everything else operates very quickly indeed. You can buy larger Flash IDE drives but again prices start to go through the roof. 256MB doesn't sound much but you can get a base Windows 98 OS install down to about 60MB using 98lite which leaves plenty of space for HTPC apps.
Jeff
Jeff