I was the same way with those games, I got DS1, got to chp 2 or 3 and stopped playing, then when I heard that DS2 was coming out I pre-ordered and then went back and beat DS1. And then when I got DS2 I didn't play that until I heard 3 was coming out....
Anyway, DS1 is an AWESOME GAME!!! It's got a good story, good graphics, it's pretty scary, and if you like sci-fi stuff you'll love it because it's basically the only good sci-fi thing that has come out on any entertainment platform in the last 10. The game has a great atmosphere and is very fun to play through. If you play any of them play Dead Space 1!
DS2 is a good game too, it isn't as good as DS1 but there's enough of DS1 in 2 to make it a good game...
Now 3.........you might just want to skip that one........ only reason I completed it is because after the first 2 I wanted to see what happened and see how they concluded the series, but the game had lost it's way by number 3. It was way too long and none of it was all that enjoyable...