You would have to check with eMachines to see what CPU's their BIOS supports. According to this spec page your machine has an Intel Celeron 440 CPU. Does that sound right?http://www.emachines.com/products/products.html?prod=W3650
The Celeron 440 is a 65nm LGA 775 chip. I would say older 65nm dual core Core 2 Duo chips would work, but eMachines would have to confirm that. One problem is the 800MHz FSB of the Celeron chip. Even older 65nm chips normally used a 1066MHz FSB.
Gaming performance is determined mainly by the graphics card. What graphics card do you have and what resolution do you game at?