First off, what kind of graphics card did you buy. I'm guessing since your said you got that from an eMachines website, you might need to buy a new PSU since most of their power supplies aren't high wattage and their crap any way so it would be recommended to buy a new one. But here's what you should do:
1. First uninstall all your old graphic card drivers, if haven't done so.
2. Restart your computer and boot into your bios. ( Look under the section Advanced and their should be something about onboard or just browse through parts of it till you find something about video) Disable it and do what tMagic said.
3. Then save all and exit.