For the real anoraks amongst us, here's a brief discussion of the that vs which question.
Use that to introduce a restrictive clause...one that tells which one:
"The car that is red is the one that hit me."
Use which to introduce a nonrestrictive clause...one that provides
more information but doesn't restrict:
"The car, which was red, hit me."
Easiest way to know which one to use: if you set it off with commas,
use which, and if you don't set it off with commas, use that.
Another sample to reinforce the idea:
"The lawnmower that is broken is in the garage."
(restrictive--specifies which lawnmower, the broken one)
"The lawnmower, which is broken, is in the garage."
(nonrestrictive--just provides the additional info that the lawnmower
---this set is from The Elements of Style by Strunk & White
Now, how strict you are in observing this is up to you and has more to
do with the tone you're trying to achieve.
If you're writing something formal and really want to be sticky, use this. For fiction, well, what's the tone of your narrator? Because I've got it firmly fixed in my head and am one of the anoraks who mentally corrects 'incorrect' usage when I'm reading, pretty much any narrator I create will follow the rule.
But in dialogue, that's another story (no pun intended!). I can have characters say whatever they want to.
In other words, there's lots of 'abuse' of this 'rule', so sometimes the advice in one of my usage books (can't remember which one) is the best: use whichever sounds right to you.