2gal is really to small, but with a sump I think it could work.Still most people reccomend nano's to the more experienced.Just came across this in new posts btw, I have even had a SW before!
Well, it's not impossible to set up tanks that small without sumps since I have a sump-less 1 gallon and I've heard of setups as small as 1/2gal (don't know if these were a success though), but you won't be able to keep much in such a small tank and you will have a heck of a time setting it up if it's your first marine tank. Tanks under 5g are considered in the pico range, and are pretty difficult to set up and maintain. I started with a 5 gallon and would really recommend looking more in the 20+ gallon range if you want an easier time with less risk involved for the livestock. If you desperately want to start small though...better stock up on books and start reading/googling to do your research, because the small tanks leave you absolutely no wiggle room for mistakes.
Er...you don't "need" corals in a marine tank. If you want them you can put them in, but you need to do a serious read through the stickies in the nano section of the forum so you know better what you're getting into.