Well... I turned on the racing line in Ass-etto Opel Corsa to know when a corner is coming up on tracks that I'm not that familiar with, I take the line and braking-points more as suggestions, after some laps I usually turn them off, cause they are distracting, but I don't know why I had them on in the video.
Well, even after thousands of laps on the same track it felt right just to turn it on and leave it like that. I know I didn't use the exact line or braking points, but somehow it made me feel comfortable and consistent. Can't really explain that. The thing that it's a constant line and not annoying arrows like in every other game makes it a lot more usable and non-distracting for the eye.
Racing line display, good or bad, can be useful for the player, especially when it's static like in LFS. The point is not "follow the line" (the line is usually wrong), it's using this feature gives you additional reference points which helps you to maintain your desired racing line. You are not supposed to follow the racing line unless you're driving a new track. You should memorize the _relative_ position of your desired racing line and brake/coast/acceleration points more accurately with the help of this display.
Does this feature improve realism? No. Is it technically useful? Yes if you're using it the right way.
Well, I got a Radeon 7850 2GB atm, and runs smootly ~80 fps@ 1080p.
So you would probably get away with a Radeon 270x, but AMD's are kinda hard to get atm, thanks to cryptocoin miners, from Nvidia, you can get something like a GeForce 760.
But mostly depends of your budget, and your current PSU.