I was in both the Warcraft 3 ROC and TFT betas and they don't just do it randomly. They want to cover a certain range of machines. So it somewhat depends on your machine, but that's about it. Last time they made you download this active x program that determined your machine specs automatically. I don't know if they are going to do that again though. It really doesn't matter because it's not like making it up will help you since you have no idea what they want.
Oh, and be prepared for cheaters. Don't be like them. These guys sign up multiple times under fake names to get it.

If they get caught they get banned.
Abs