I put RO/DI water right up at the top of the list of must haves.
You may be licky enough to have decent tap water but probably not. even if it is good drinking water does not mean it is reef quality water.
When buying an RO/DI unit don't fall into the number of stages game. the number is irrelevant, its what is contained ih each stage that matters. For RO only you need 3 and RO/DI you need 4. 5, 6 or 7 stages doesn't mean a hill of beans if they are things like granular activated carbon (like the Aqua which I would not recommend) or a post RO drinking water taste and odor cartridge.
Some of the units I will recommend here also use multiple carbon blocks which research has shown are not really necessary but for the price and quality they would still be a best buy in my book.
www.spectrapure.com - MaxCap - By far the best RO/DI unit on the planet.
www.buckeyefieldsupply.com - 75 GPD Premium - Great unit and very affordable including all the bells and whistles to get you up and running. The only thing I would add is an autoshutoff valve.
www.thefilterguys.biz - Ocean Reef +1 - Great unit also. I would probably opt for a handheld TDS meter over the inline variety as they are more accurate but to each his own.
www.melevsreef.com - Reefkeeper- Very good quality. Marc is a frequent contributor to RC and a very knowledgable individual. This is the only 100 GPD system I would recommend but it is because it uses a different brand of RO membrane not the 100 GPD Dow which i would not recommend.
www.airwaterice.com - Typhoon III- One of the most popular units for years.
www.purelyh2o.com - Optima series - there are several 75 GPD versions in the Optima series depending on how you intend to use it. Some are easily automated others are more basic.
Whatever you get, buy yourself a good handheld TDS meter as you will use it a lot and come to depend on it.