I've owned several SKSs, and my best shoot with an SKS was a 5 shot string grouping 3" at 100 with a Russian made SKS. The Yugo was about the same accuracy, and the Chinese slightly less (could be me, but my M14S groups 1 1/2" at 100 with standard sights, no problem).
The sights are fine for 100-150 yards, althought if you're going to mount a scope have a proper mount installed on the side of the receiver, the aftermarket dust cover replacment mounts have a reputation for shifting a bit, and losing their zero when the rifle is broken down for cleaning.
For what it's worth here are the Remington
ballistics charts for 7.62x39mm