From my own experience and articles I have read, most flight sims push the processor more than the video card, unlike many of the new games. Thus you could save a little on a video card, and put the money into a faster processor and RAM. Some of the scenery add-ons also recommend more than 512 MB of RAM, so you should look at what you will be doing with the fight sim too. Check out
FlightSim.com and
AvSim.com, they'll probably have more info about what a good system for flight simming would be.
goates