You aren't by chance trying to run FS9 in SLi mode are you?
BTW I have found that most drivers for the 7800 don't work nice with FS9, most of the newest ones fixed previous issues.
I had some crashes at first and then corruption of graphics in sim, such as building textures covering the runways lol.
Now am using the 82.12 drivers and FS9 works wonderful.
And for the ram, I had two gigs installed but removed one stick and FS9 runs better, may be due to the fact that Windows doesn't know what to do with more than a gig

has a bunch to due with memory management and page file stuff I wont go into becasue I confuse myself most times LMAO!
4 gigs may be something for Vista when it comes out but IMHO overkill and wasted money for XP.
But I say it has more to do with your two cards and Im thinking you might be trying to run in SLi, I have managed using both cards in FS9 with some nice improvements but using the second card for SLI AA, but overall better performance I run in single card mode.
SLi rocks in all other games! CoD2 for example is amazing!