On my system, I'm running it at 1600x1200, with everything except autogens turned to full and I'm getting a constant 20FPS.
And Tim, you'll also notice that the 7800GTX is more expensive on some sites than the 7900GT, even though the 7900GT performs a bit better.
With FSX, I feel that more RAM (at least 2GB) will make a bigger difference than the GFX card.
AMD64 X2 4400+
DFI Lanparty Ultra-D
2GB Kingston Hyper-X (2-3-2-6)
TT Big Typhoon
eVGA 7900GT KO Superclocked (650/1650)
250GB Maxtor DiamondMax SATA2 HDD
Hiper Type-R 580W SLi PSU
19" Sony Trinitron CRT