With so many different versions of Street Fighter to play, it's understandable if you've gotten confused along the way. The new add-on for Super Street Fighter IV, Arcade Edition, is set to release next month, and a Q&A posted on the Capcom Unity blog explains, among other things, how things will work for those who do and do not want to purchase it.
The Arcade Edition DLC will bring identical character balance to that of what you see in actual arcades, as well as four new characters. It'll be available on June 7 to owners of Super Street Fighter IV (but not regular Street Fighter IV), but in order to purchase it, you'll first have to download an update that will also be released on June 7 for Xbox 360 and PS3. This prepares the game for one of two DLC packs that you can then download.
If you want to buy Arcade Edition, you'll be able to do so after getting the patch for the price of $14.99 (1200 Microsoft Points). Alternatively, you can download the free "Update Kit" that will enable you to spectate matches using the new content, and allow you to play against Arcade Edition players. Matches against AE players will be restricted to old characters; the free DLC won't allow you to play as or against any of the four new characters.
In addition to the downloadable version of Arcade Edition that requires you to own SSFIV, Capcom will also be releasing a disc-based version with everything included for $39.99 on June 28. A date for the PC version -- a platform that never a saw Super Street Fighter IV -- is still undecided, but "the earlier part of July" is currently the goal.