Thrashing will wear drives quicker due ti increased heat and the physical vibrations caused by many rapid movements but it's not somthing that I'd be worrying about on a desktop. The increased wear is only minor compared to "normal" usage. Unless your running a huge database with a few thousand users......

Bob's right about some of the hard drive coolers out there and power supplies. Getting good quality for both will make a huge difference to your whole system, not just your drives.

Also remember that drives have a reasonable life time but will die eventually. I've still got a 2.1Gb drve that have been used heavily for over 10 years without issues but had a 120Gb only last barely 2. BACKUP anything you value even slightly, prefferably to another physical system.

BRETT