reef tanks are great, and are very addicting. They do get alot more expencive than anticipated though because every time you go to the fish store to get something, you walk by the coral and see just the perfect frag for that empty spot you niticed in your tank, and as luck would have it, it's on sale for just $20. so obviously you get it, and perhaps a few more snails to help with clean up, since they are only a buck or 2 each. you got what you came for, a new coral frag, some snails, and on your way to the check out counter you see a sale on fish food, and you're gonna need some kinda soon anyways, so you grab that too. before you know it you just spent over $100 when you intended to spend maybe 1/5 of that. Every frag i get and snail is cheap for how much I'll enjoy it, but add everything together over time, and it's not so cheap anymore. Of course I love it and wouldn't be without a reef tank now. It can be very rough on the wallet if you aren't prepaired for it though.
A far as the tank set up you picked, it sounds good to me. you'll always want a bigger tank, so after you get yours pretty full of fish and/or coral you're gonna have the urge to upgrade. Doesn't matter if you start with a 20 gallon you'll want to upgrade to a 40, or from a 40 to a 75, or from a 75 to a 120...etc. My corals are all still frags, and I'm already thinking about my next tank. I'm gonna try to skip the in between tanks and go straight for a 200+ gallon.
Ohh yeah, and if you're looking into starting a reef tank, seriously look through craigslist in your area for tank set ups. the price drop on a used tank vs new is astronomical. For what I spent new on my 40 gallon i could have gotten a used 120 in good working condition. Just my 2 cents.