Graphics card upgrade help?

I've got an http://www.asus.com/Tower-PCs/CM1855/specifications/
Radeon HD7770
16GB ram

I wanted to upgrade and was wondering if anyone could tell me how large a graphics card I could get? Gaming specific? Without going over $2-300? I'm assuming the card I have is not optimal. It's sufficient and it is the onboard sound that gives me more issues than anything, but, I wanted to do a one time upgrade and hold on to the thing for a couple years. Maybe enjoy BF4 a bit more.

Thanks for any suggestions.