FSX doesn't care that much about the GPU itself so don't expect huge gains when switching.
Substanitally more memory on-board
HMDI(mini is okay) output
Something really fast
Can be overclocked safely if needed
You may look for the still current Nvidia GTX460 or the newer 560 cards. They clock well, the run relatively cool and on a lower power consumption than their bigger counterparts and they also have some variation when it comes to different memory amounts. Pick 1gb or above.
Look closer on the actually model before buying since some devs use their own cooler designs, offering only other optics or an increased usability (low noise/better cooling), it really depends. The optical stuff isn't of any use and sometimes even louder than the default design.

The 460 should come in a bit cheaper than the 560 but won't lose much (if anything) in pure FSX terms. So your wallet may decide.