Emachines usually use a standard ATX or mAtx Motherboard how many expansion slot does yours have?
One word of note though it's usually just worth it to buy a cheap case and not have the problems of front panel wiring and fitting the board. >
http://www.newegg.com/Product/Produc...82E16811147057
Also if you change the motherboard you will need a new copy of Windows if yours is the OEM copy supplied with the pc according to the EULA >
http://www.newegg.com/Product/Produc...82E16832116485
Will will also need at least a 650w power supply for the 8800GT. >
http://www.newegg.com/Product/Produc...9005&Tpk=650tx
Motherboard >
http://www.newegg.com/Product/Produc...82E16813128347