Once upon a time I loved playing Street Fighter 2 (turbo, championship edition etc) on SNES and in the arcade. I'd like to give SSF4 on XBox a chance. What advice would you give to someone in my situation? My favorite character(s) were Ken/Ryu. I'm interested in basic strategy and moves (fireball, upper cut etc).
If you haven't played any Street Fighter game since the Street Fighter 2 series, there are a few concepts to learn. The game overall feels more like Street Fighter 2 than most fighting games that have been released since then, so you will be in a familiar mindset. Using fireballs to zone your opponent to keep the distance still works, as using dragon punches to punish random jump-ins.
There are some new mechanics, though:
There's a set of video tutorials aimed at beginners:
Ryu and Ken were nearly identical to each other in Street Fighter 2, but they became a lot different since then. Ryu is best played by using fireballs to keep the distance to the enemy and punishing their approaches, while Ken is best used in rushdown.
The basic moves have not changed. The largest changes are the introduction of EX, Super, and Ultra moves, as well as concepts like "armor-breaking attacks" and cancelling out of a move as a form of feint. If you still remember the SF2 tactics and moves, you'll want to concentrate most on learning all of the advanced moves so you can integrate them into your fights. Otherwise, you'll get trounced pretty significantly by someone who knows how to use those mechanics.