Thanks for letting me know 'bout this thread. I haven't had much time to keep up on the other forums as much as I'd like lately.
I've kept saltwater tanks for upwards of 10 years now, have worked in the trade before, and currently run the photography forum at reefs.org, a great online resource for the hobby.
Let me first give you some bare-basic info, and then we can go from there.
First, let me dispel a couple of rumors. There's no real magic to keeping a saltwater tank. They are
not more difficult than freshwater, it just takes a lot more learning to understand all the biological interactions that are going on in the system, not only between the various animals (fish, coral, etc.), but also at a microscopic and chemical level.
A 20 gallon is a decent size to start with, but there are a couple reasons why I"d lead you away from a 20.
1. Bigger is better. As water evaporates from the tank, it's only
water that leaves, not salt or other chemicals. Lose a gallon of freshwater from a 75 gallon tank, not that big a deal. Lose a gallon of freshwater from a 20 gallon tank, that's a substantial portion of your water volume.
2. 20 gallons may look big, but you'll outgrow it
very soon. In a freshwater 20 gallon, you can pack 20 inches of fish into it. In a saltwater 20 gallon, you can have two,
maybe three small fish.
Next big thing is to decide what you want to do--fish only (FO), fish only with live rock (FOWLR), or a full-blown reef tank. There are some slight, some major differences between the different types of setup.
Most notable difference is in lighting. Corals need massive amounts of light. A decent lighting setup for a FO tank can be had for under a hundred bucks. A decent lighting setup for a reef tank of the same size could very well set you back a grand or more.
The best advice I can give you is to read, read, read. That's your best defence against failure. I'll point you to a couple of really good websites here, and feel free to post here or email me with more questions.
www.reefs.org [Great online web community, set up with the same php software as this site, so it'll look very familiar. There's also a 'new reefkeepers forum' for basic info, plus lots of other discussion areas (and the photo forum!!

]
http://portfolio.iu.edu/jwikman/finalsite/index.htm [This is a site I built for a college class a couple years ago. It's a good all-in-one reference for people wanting to get into the saltwater aquarium hobby. Tons of info about every aspect of keeping saltwater systems, as well as links to good books.]
www.advancedaquarist.com [This is an online magazine devoted to keeping saltwater aquarium systems. Some of the articles are pretty high-end, but you can learn
tons reading it.]