Skyrim experiences

So i just started playing Skyrim on my DS computer with core i7 @4.32 GHz, 16 GB DDR4 2888MHZ RAM and GTX 980 Ti Asus Strix Edition. I just got my new Acer XB270HU 2560x1440 monitor @ 144 Hz.

It seems like Skyrim was definitely not designed for advanced monitors, because I had to set the game to a strange resolution, 3620x2036 DSR. (on max settings of course)  to get it to work properly. At 2560x1440, the intro Helgen scene would glitch and the horse carriages would fly around randomly.

The monitor was set to 144Hz when I had it at 2560x1440(which is when it was glitching) and it was set to 120Hz when I had it at 3620x2036(which is when it worked fine). Does anyone know why the game is like this? Its rather strange. Does it have to do with the refresh rate or the resolution? or both? How do i know the game is actually running @120 Hz?

Oh, and lastly, could anyone kindly point me in the direction of some nice Skyrim graphics mods, so I can make the game look even prettier than it already does?

And by the way, I have a hunch that if you buy games from Steam, they are different than if you buy them out of the box. Is this true? I remember in the past hearing negative things about how Steam treats certain games rather than if you install them from the CDs, Maybe I'm just having a delusion, but i'm not sure.

For the mods, I always head over to Nexus.