Agreed. Think of games you want that have yet to come out, that may require that little extra you put in now. It will save you money down the road.
Also agreed. The very first computer I ever owned I built myself with help from people here and the internet. Anyone who can read and knows how to fit Tab A into Slot B kinda thing can do it. I ended up with, for the limited funds what was a fairly decent enough computer.
You will definitely get more bang for you buck doing it yourself. Well worth the effort! Especially if like me and learning new things is fun, whatever they be.