Also, from what I've seen a by-pass (literal meaning is the same thing) is used to describe the type of valve that does the same thing but does not blow off into the atmosphere (blo-off or pop-off) but instead redirects the air back into the intake. This doesn't make the hissing sound, so it is used on all the new factory turbo set-ups, especially by VW (the 1.8t and Audi TT), they work just as well and sometimes better when tuned correctly, but who wants to give up that awesome sound?
But anyway, if you have the money, get the blow-off valve and the turbo timer, in that order...they are both good things to have, if not just for the piece of mind.