Flight Sim X and Graphics Cards

I have a Antec 900 with a Q9400, 4gb of ram and a 9800gt. I get about 27FPS on FSX. It looks decent but i want better scenery. Every time I pump up the scenery it starts to lag. Everyone says this game is a CPU intensive, but my CPU doesn't work very hard. I think it the graphics card. Any recommendations for better performance.

I've been looking for the answer aswell.. Don't go with either of those cards. From what I've read FSX prefers Nvidia, specifically the pre-200 series cards. I just bought a 9800GTX+ (haven't got it yet but it's apparently one of the best cards for FSX) although I got a bit screwed cause I found out later that a 250GTS is just a rebadged 9800GTX+ that runs cooler. So I actually paid more for an older/hotter card. Also I assume you have SP1 installed for FSX, otherwise it won't take full advantage of your quad core.
-Adam