We can agree that a simulator is not a mere game (
with Flight being the classical exception that confirms the rule, it being a mer[d]e game sold as a simulator) and for a simulator... WHATEVER THIS SIMULATOR THIS MAY BE... upgrading the sole video-card is like... mounting an oversized engine in a car. Unless you tune up a lot of other pieces, you won't see any real advantage out of it. 8-)
These other pieces are the CPU and memory, this last both in size and speed. Of course a faster HD helps too... and so does an audio-card with a self reliant chipset that does not use the PC's CPU to process audio data (
a plague, lately, these audio interfaces that are dumber than an old ISA connector SoundBlaster 16 of all the way back in 1994 and only to save the 50 cents of the cost of a sound processor)... :-/
There's no dearth of pieces to tune up to the new video-card's real or supposed power, anyway. Coming myself from a Radeon 9200 (
back in 2004), I can attest that almost any other more advanced card is better. Of course, changing only the video-card hardly makes miracles in the the betterment of a sim's performances.

There is no such a thing as overkill. Only unworthy targets.