I take it that, that is a X300, for your machine spec something a bit more meaty is required. It is basically throttling it.
I have just ordered an X1600 pro, to replace the 9600 (AGP). Machine is an AMD 3200/64 1gb ram.
Even the X1300 would be better than an X300.
Not yet tested on FSX.
My first sortie - only about 45 minutes, I was getting 15-16 fps "out of the box" 3.2 Ghz, 1.0 Gb Ram Graphcs card is a Radeon 300 128Mb