Its pretty easy to overclock the newer HD graphics cards with catalyst control center. I unlocked mine so technically it's void even though I haven't needed to up the numbers any. And overclocking the graphics won't affect her MOBO/chip warranty.
She added internals to hers herself, so this doesn't apply. But Cat, if you buy your desktop computer from Dell or HP or Gateway and you crack open the case, just to see what power supply you received or to add another HD or put in a TV tuner card, it breaks a little seal when you pop the door off and technically you voided your warranty with them if they want to push the issue. "Play by the rules, you'll miss all the fun!"