Opening up the box is nothing to worry about at all. Often when installing new hardware it's not even necessary to rtfm, and this is especially true for...uhm, almost everything.
I check the instructions if there's a jumper that has to be moved or something - almost all other things install by pure intuition.
The only thing to keep in mind is to make sure one is properly grounded (and that's quite an important thing too).
And no, I would never buy a pre-assembled computer.