Super Street Fighter 2 X/Turbo was the final update in the SF2 series. New additions were the Super moves, Akuma as the final boss, faster gameplay, new moves for old characters, and increased difficulty.
Home versions of SSF2X included Dreamcast, PS2, and XBOX. The two latter ones were released as part of the 15th anniversary of SF2 and enabled players to play sf2 characters against their ssf2 counterparts. This was really exciting...honest!
The Dreamcast version of SSF2X was released in very limited numbers and commands a high price. The Dreamcast version also has the hidden dipswitch menu, which enabled players to alter text/colour/release versions and also enabled Shin Akuma/Gouki and Ten Gouki/Akuma.
Super Street Fighter 2 X remains an excellent game even today. It has aged fairly well, but those coming from the recent VS and Alpha series will find SSF2X's difficulty less of a learning curve, and more of a learning right angle!
The most exciting aspect (story wise) is that Akuma/Gouki is credited with the death of M.Bison/Vega. However that doesn't mean that Capcom won't find a way of reviving him.