Verified answer

The el1358g specs sheet:

http://support.gateway.com/emachines/desktop/2011/...

You didn't list what you currently have so it's difficult to make a judgement as to whether or not you could or should attempt to upgrade. Quad-core doesn't look like it was an option, single/dual/triple core appear to be supported, specific ones anyways.

Your Pc can be upgraded to the AMD Phenom II X4 955 3.2 ghz processor. That's practically the best up grade for that motherboard. That processor will also be relatively good for gaming. I own the emachines el1358g and I upgraded to the AMD Phenom II X4 955 3.2 GHz/6 MB L3/125W Processor

http://www.amazon.com/gp/product/B00275G0VE/ref=as...

I also upgraded my Memory to 8 GB DDR3

I am hoping that you have upgraded to a good graphics card as you stated blatantly above.

With my current specs, I can play most of the modern games on even high settings.

AM3 is the support of your motherboard so you can get any CPU that has AM3 support. But I don't know how much WATT your PSU have. You can get a AMD Athlon II X4 Quad.

I don't recommend upgrading it if your PC is crappy just get a new one.