Since you got lots of RAM capacity, it might just be the graphics card that's bottlenecking your PC.
Since you came from PS3, I expect that you are playing on a full HD meaning 1920x1080 resolution. If you do, you seriously have to get a graphics card better than ATI HD4770 like an HD5770 or HD4870. But if you do, you will need higher capacity PSU and more powerful processor so you might as well replace the PC altogether with a new build and keep the Aspire for light PC use ( to save electricity ).
However, if you are willing to play at resolution 1360x768 or less, then I think the HD 4770 will do and you might not even need to replace the CPU.