Hey Al... might I suggest you take a trip over to the sister site http://www.backyardherds.com and wander around the goat section for a week or two. There are a LOT of active "goaties" over there and any one will be more than happy to answer questions or try to persuade you on why their breed is best. Lots of good folks over there too. Several things to consider before purchasing your dairy goat(s) #1 they are a herd animal and so should have a same species companion.
You'll want to determine how much milk you want per day, and how many months you'll want to milk. Most breeds production decreases over time, so though they may be producing gallons right after weaning the kids off her, that amount will normally drop off over time. Most goats are normally only milked for about 10 months, then dried off so they can be re-bred. Some goats can be milked for up to 20+ months without being dried off/re-bred. Some goats only breed during the fall/winter season while others can be bread basically year round. So if you want continuous milk, you need more than one goat so some can be milked through while others are re-bred so they can take over when the others are dried off.
Generally speaking, the full sized Sannen goat is the highest quantity producer with some animals producing up to and over 2 gallons per day. The highest butterfat milk is most commonly attributed to the Nigerian Dwarf, but as a smaller goat, it won't come anywhere near 2 gallons a day and generally maxes out at a gallon or less per day. A compromise is the Lamancha which has high(er) butterfat and high volume, a gallon or more per day is not unusual.
One of the areas that are gaining in popularity is the "mini" breeds where a Nigie buck is bred to a full sized dairy doe to produce mini breeds... mini mancha, mini nubian, mini alpine, etc. The biggest advantage is a goat 1/2 - 2/3 the size of the full size dairy doe that will produce almost the same amount of milk as the full sized on a diet that is about 1/2 what the full sized goat needs to consume. Another benefit is obviously the space requirement to keep them is smaller.
There are other breeds as well such as the Oberhasli, Toggenburg, etc. but the most popular dairy breeds seem to be Nigies, Lamanchas, Alpines and Nubians.