My BF is a big Star Wars fan and he and I disagree about how you should watch it. His suggestion: Watch them in order (episodes 1, from 1999, through Episode 6, from 1983). That way, you get the story of Darth Vader, which is what it's all about.
He said, though, that you should be aware that:
Episode I: Stinks!! LOL--I agree with him
Episode II: Somewhat better
Episode III: Supposed to be pretty good, but we haven't seen it yet
Episode IV: from 1977; it rocks
Episode V: Awesome
Episode VI: Starts going downhill with cutesy marketables.
I disagree about watching them in order. Personally, I'd watch them in the way that they were released--4,5,6, then 1,2,3. My reason is that the older films were really great, but the technology and special effects are old. I am thinking that you may be disappointed if you watch the new films first and then go on to the old ones. Also, for me at least, 1,2,and 3 didn't have the appeal that _made_ me want to watch the whole series. With all their CGI and special effects and "name" actors, for me they were a big disappointment. On the other hand, 4, 5, and 6 were incredible, and I feel that if you watch them first, then you'll want to know the "back story" and will put up with all the garbage in 1,2, and 3.
So there--two different opinions and I've been no help to you at all. }(