At the least, play Chaos Theory and Double Agent first. It's very hard to go back to the original game and Pandora Tomorrow has a few big snags in gameplay.
I still think Chaos Theory is the most replayable of the series. I nearly 100%ed Conviction in less than a week without really trying that hard; it's a great ride, but screams to be rented now; until the price drops.
"I assure you, whatever the others promise to do, when it comes to the showdown, they won't be there."