Yes I think surround is worth it. The difference between simple stereo and 4-channel surround is not necesserily spectacular, but still significant. It increases the sense of envelopment. It is more like the sound is coming from everywhere, which resembles the situation in a reverberant church room.
The quality of the sound from the rear does matter. If the quality and/or the placement of the rear speakers is poor, in my experience you cannot increase the volume too much, or the effect will become irritating. So with optimal speakers and placement, you have more freedom to increase the volume of the rears and the resulting amount of envelopment. This is especially relevant in sample sets where the balance between front and rear can be adjusted with a slider.
You mention an (extra) amp. I think nowadays active speakers (which don't need an amp) are superior to passive ones: for less money, you can get equal or better sound quality. I recommend Genelec, but of course many other good brands are available.
I recommend nearfield speakers, to be placed at equal distances in a symmetric setup, distance to your listening position about 1 m or less. This way, you will minimize the negative impact of room reflections.
I have not very extensive experience with subwoofer, but in theory, multiple subwoofers will help to even out the peaks and nulls in the frequency response, so it can be worth it to have multiple ones. Optimal placement for subwoofers is typically in or near a corner.