As log as the system is properly cooled leaving it on 24/7 doesn't matter except the cost of energy. I set my Vista systems to 'shutdown monitor' then 'sleep' after a period time. Touch the mouse and the system comes up full power instantly.
if you are constantly downloading content from the internet, you can just turn off the monitor and the video card and leave the disk´s on. the cpu wont get hurt if you have good refrigeration because when in idle it will down throttle.
An OC would produce more lost energy should you let the system running 24/7 without monitor/system power down phases or system apps like Speedstep/Cool and Quiet. Leaving it on 24/7 is fine, but it cost money.
For the past 15 years I've always turned off my home PC at night, and I never turned off my work PC. Guess what: both were just fine. If you have a good reason for the PC to be up (downloads, server stuff, etc.) and the noise/lights don't keep anybody awake, it's fine to leave it on. If not, it's fine to turn it off.
If you leave it on and have a power-hungry gaming card you could also use nTune or Catalyst to underclock it when you're done gaming. That should save you a bit more in electricity bills. Also, you can select a power scheme that turns off idle hard drives after 20 minutes or so.
The one issue with systems that run 24/7 is with the mechanical devices such as fans and hard drives. When restarting a system that has ran for an extended period of time, you run the risk of the mechanical part not working properly after the restart. The company I work for has experienced the majority of drive failures after downing a server that has been running for long periods of time. While equipment will run and run and run, stopping it and then trying to restart it is problematic.