cool, well first of all that link above is good reading as far as the skimmers go. I personally dont feel confident in my knowledge of skimmers to make one, but its an option for those who are that way inclined.
imo a skimmer is a great investment for a marine tank, i know a guy here in hamilton that ran a big aquaone tank stock and it did ok as a fish only tank but none of his corals prospered due to water quality issues and the level of light. my advise to him was to get rid of the stock filtration as it is more suited to fresh water, and to put a hang on the back skimmer on the tank.
other than that it depends on what your plans are for the tank. Full reef, fish only, soft coral tank, hard coral tank. the gadgets you need will depend on your preference