I can not find recommended requirements for FS9, but CPU wise you should pass easily, how is the game now, I mean FPS, is it choppy.
Without going into details you can NVIDIA GTX 650 (FSX reported to behave better with NVIDIA, than with ATI)
This ASUS GTX650-E-1GD5 GeForce GTX 650 1GB
http://www.newegg.com/Product/Product.aspx?Item=N82E16814121710 card will work on your existing power supply, do you have Mini Tower
Check the picture below, it is not yours PC but similar, tell me which form factor you have
If it is MT, my recommendation above is excellent, if it is Slim, you need low profile card.
Since you are using old PC and old Flight Simulator, I would recommend upgrading one part at the time, because next would be CPU.
Anything else I can help you with?