NOTE: If you're going to spend the money on a high-end gaming PC, take the time to find a local nerd that you trust and ask him (or her) what you should buy! Don't trust the retailers! They're not there to help you, they're there to sell products. Find someone who isn't directly affiliated with a retailer and ask them the questions you want to know!
There's only two types of games where the mouse and keyboard setup fails miserably: racing games and flight simulators. However, this shortcoming can easily be rectified by buying a Windows-compatible Xbox 360 controller and plugging it into the computer with a USB cable.
(For those not tech-savvy enough to understand what that means, it means you can use a controller on the PC as well.)
If parents want to give their kids a PC that can run the newest games at a decently high graphics level, they're going to have to be prepared to drop at least $1000.
This means that if you buy a PC today that can run an older game like Far Cry 2 at Ultra-High settings, your PC will be nearly useless in about 3 years.
There are some systems that combat these flaws. Valve's anti-cheating software is fairly stringent, and any hacker who gets reported to Valve usually has their Steam account banned. It's not a perfect system, but it's better than it used to be.