[jester710]

You need to try to track all the cards because knowing what's dead could allow you to limit your opponent's range or better estimate his chances. For example, if your opponent's betting out with a 3-flush showing and you know that a lot of that suit is dead, it's less likely he has the flush. Whereas if the suit was very live, you could probably save your money.

One trick to remembering cards is that you only need to remember what's been folded. And for the time being, if you can remember either rank or suit pretty well, I'd go with remembering rank and just trying to get a good estimate of suit.