• For your first question, HDD cache is basically a high speed "holding area" for file transfers. Here's a quote on it:
A cache is used to speed up data transfer and may be either temporary or permanent. Memory and disk caches are in every computer to speed up instruction execution and data retrieval and updating. These temporary caches serve as staging areas, and their contents are constantly changing.
If you do things that require access to multiple items consistantly, like a game with many load screens, or video editing, etc... you'll notice an increase in those load/transfer speeds ever so slightly. (in my opinion)
• For your second question, here is a link to a review of 22 hard drives from multiple manufacturers: http://www.tomshardware.com/2005/09/27/round/
Hope this helps you out!