Changing to 2GB 660 will not change performance. You need 760, 770 for better FPS. You can probably run 760 with the current Antec. The CPU is ... decent.
For 770 - probably new PSU.
But this is office machine, If you plan serious gaming better design a gaming machine from scratch. Case, MB, GPU, CPU, RAM, PSU etc. All components must be planned for gaming in advance.