no need to upgrade your RAM. 4GB is plenty and 12GB is pointless, there is nothing out there that needs that kind of RAM. your CPU is fine but what's killing your PC is the ATI 4650. It's an entry level gaming card from three years. if you have a 300w PSU then get an ATI 5670.
If you want more performance, post your power supply (make and model) , upgrade budget, and monitor resolution.