ya, with prebuilts you really can't swap anything out without voiding warranty.
If your doing heavy gamming sometimes a computer will freeze or shut down because of overheating or there being insufficent RAM or the setting are to high, you get my point.
After playing farcry for awhile on high setting my video card will die on me and reset itself I still need to restart though. It happens. My issue is related to heat, my room gets really hot.