There's alot of fans who will say the previous Batman films are crap. And that Tim Burton ruined Batman and yadda yadda yadda, but I am not one of them. I believe Burton did a amazing job with Batman 1989, and I actually know some fans who prefer Batman Returns! Schumacher in his own right, is a pretty good director as well. However, WB's direction and his love for neon and Bat-Nipples ruined any chances for longterm success with his Bat-Flicks.
But yeah, you will hear from some who say the Keaton/Burton Batman films are shit, and I have little doubt the same damn thing will happen 15-20 years from now when WB decides to reboot the Batman film franchise once again following Nolan and Bale's series of films. It's an unfortunate cycle, but that's the way it goes.
With Batman Begins, I highly recommend you check it out before seeing The Dark Knight. I dont really believe it's required, but it certainly would help give you some idea of how Nolan's vision of Batman differs from Burton's and what not. Ra's is of course the main villain in Begins, and Cillian Murphy does an excellent job in bringing The Scarecrow to life (who also appears in The Dark Knight as well in what is called a glorified cameo). For my money, Begins also has probably the BEST ending to a superhero movie as well.