Under low settings, that setup should be fine, but that can't handle high settings in Flight Sim X. Flight Sim in general is a choppy game, even high end PC's don't get very good framerates.
To play that game on high settings, you're gonna need almost an entirely new PC.
Processor - The one you have now looks ok, but get a Core2Duo if you wanna upgrade, those are the best kind out right now
Motherboard - Get one that has a PCI express slot, this will enable you to buy the latest graphics cards. (and obviously is compatible with your processor)
Graphics - The 7600GT is a good deal:
And you'll need at least 1 GB of quality RAM.
If you just wanna try to upgrade this PC, check out this graphics card: