Ahh right. Emachines probably has a shitpowersupply that will die with a high-end card. Try to find out if that model has a standard ATX power supply, if it does you can pick up a Fortron made power supply for pretty cheap, and it'll handle anything you throw at it as long as it doesn't involve SLI.
If it's not standard ATX, well you'll have to look into the lower power consumption market... I'm not sure how much power the 6600GT consumes, if you can't change power, I think the best you can go with is probably the 6600GT.