I don't recommend using a global $fn, 100 faces may be too much for a small circle and not enough for a big one. $fs and $fa scale much better. $fs sets the maximum length of a segment before it starts a new one, $fa sets the maximum angle it has to turn (i.e. to follow a curve) before it starts a new segment. I personally write my modules to use something similar to $fs because I find that scales best, all the objects look like they are the same 'resolution' whereas $fs and especially $fn can leave small objects overly detailed and large ones looking blocky.

Also, OpenSCAD forum: http://forum.openscad.org/

One thing I love about OpenSCAD is the idea of modules. You only ever need to solve a problem once. There's much, much more so if you're interested I will preach. Also, you can do things like chamfers and fillets, they just take a bit of pre planning, so don't let that put you off.