For a game like FSX you primarily will need a good GPU with a memory of 1gb or more...(games now most likely takes the power out of GPU so thats what really matter) and at least dual core clocked at 3ghz+.
As for Games . You may consider the High-End ATi HD 5850 for a better future-proof gaming experience or even a gtx 470.( recommended if you have a good airway flow/case and PSU).
And as stated above. A 4gb Ram will be enough.