I don't know two many tweeks but here are somethings that might speed up gameplay

First of all if you have the omega Drivers (extremely moded/tweeked out drivers just in case you dont know) there are some motherboard tweeks that are on the omega website over on the nvidia section.[I don't know if you could use them without the Omega drivers
http://www.omegacorner.com/index_nvidia.html (click on Mb Bios on the side)

I haven't tried um yet and I havent gotten to play my 5600 yet, (I was faulty so I had to get it replaced) so I havent tried any of them yet

ohh and If you havent tried the Omega Drivers you might want to try them, The Drivers for ATI really made my card faster, but I've never tried the Nvidia ones, Its worth a shot, plus you get to use the mobo tweeks.

You might want to shut down any background applications (eg firewalls, popup blockers, etc) and maybe even turn off explorer while you play (IF you have XP start up FS9 quickly alt+cntrl+del while splashscreen is on go over to proccesses and then end task on explorer) If you do it fast enough you will end explorer before you start FS9, and have something to do during those few seconds when FS9 is loading

I'd like to also refer you to this post, http://forums.avsim.net/dcboard.php?az=show_topic&forum=121&topic_id=136178&mesg...

The little fix increased my FPS by 7-10fps