theres a great bit of info about skimmers at
http://www.hawkfish.org/snailman/skimmer101.htm the same guys also has instructions on a DIY skimmer that Igerold made and that im currently working on.
If you wanna go the DIY route, itll be more fun and scary... but less expensive! about $30 - $50 for plumbing and another $50 for a good airpump. you can always make more than one, or make one oversized in order to facilitate future upgrades.
ok other than that, my only skimmer experience is with a cheap $30 skimmer that works just fine on my 30 gallon system.