IIRC FSX works better on quad-core CPUs than on dual cores. Think about a MB/CPU upgrade. Adding more RAM could reduce the choppiness too. You need a 64-bit Windows to take advantage of more than 4 GB of RAM.
The GTX 480 is several times faster than what you have (http://www.tomshardware.com/charts/2011-gaming-graphics-charts/Metro-2033-Enthusiast,2664.html),
but there are better options now, e.g. the GTX 770.
If you do buy the GTX 480, you need a PSU with at least 550W.