Much depends on what CPU you have, and what graphical settings you want to use in game, but there's plenty of posts in here from 560 users reporting that whilst it's a grand GPU, in Railworks it performs not much better than its (cheaper) siblings. The 550 seems to give performance as good, a 460 (if you could find one) would, and a 450 would appear to come close.
High frame reates aren't particularly important, what you should be looking for is a constant rate---obviously a constant 14 isn't going to be as good as a constant 25 but anything over that isn't going to improve your enjoyment by much.
I have a 550, helped along by an i5-2500 CPU admittedly, and with graphical settings up as high as they go (apart from particle and passenger density) Anistropic set to 8x, using SSA 2x1 and a resolution of 1920x1080 I can get close to your figure of 50FPS on a very lightly populated (scenery wise) route....
With the frame rate set to 30 using the command line option the game looks, nor performs any better.
Of course if you use other games that will benefit from a more powerful card then it's a good choice.