Of course, two things cause this:

1) Software (Drivers, don't use the ones that came on the CD. Use the newest ones from the manufacturer's website. Sometimes the authentic NVIDIA/ATI from nvidia.com/ati.com won't work and you'll have to use the manufacturer's specific driver.)

2) Hardware (Heat. If the "artifacts" don't happen when you have the graphics settings cranked down, but they happen when everything is cranked up, then it's a heat issue.)

If you can diagnose whether it's hardware- or software-related, then that's 90% of the battle.