Gaming System Software Image

[OblivionSHO]

I have a few questions for the fellow geeks out there.
I am building a few gaming machines for my house. These machines will pretty much be purely for WoW, UT2k4, etc.
Now the questions. What software do you think would be best to keep these machines up and in good running order, but not slow them down that much? I have heard things like AV software and certain disk defragmenters can actually slow the system down a touch. These machines are behind a hardware firewall as well.

Opinions please...

Thanks.