Running fs9 with 512 of memory is fine,with one card and reasonable video settings, but remember
the PC will use onboard FX5200 memory and system memory.
This version of FS is the most resouce hungry yet.(try one of the free memory cleaners and watch your system ram drop like a stone when you fire up and use the sim)So more ram would help,and is cheaper to try than buying another graphics card
If you are running XP and your mobo supports it,try more ram
I used to run xp2800 with a FX5900GS and 512 mgs ram with only one monitor and had stuttering and flashing graphics if I pushed the settings too high.Ive now added a stick of 1gig(=1.5gig)and with all settings at max I have no problem
A lot of people were dissapointed by the perfomance of fs9 when it 1st came out,because not many of the flight sim players had ever thought of running more than 512gig ram(which was above normal for PCs at that time anyway)
One last thing,the FS9 fix (MS called it the update) have you installed it?as it fix's a problem with autogen scenery(used to hog memory)
Hope it helps
Let us know your thoughts