When do I resign?

[FuzzieAlum]

I'll be going back to graduate school this fall, which means I'll be quitting my current position. How much notice should I give my employer?

I've read that I should just give them two no matter what, but they'll know I could have given them much more - no one decides spontaneously to go to graduate school two weeks before it starts. I don't want them to think I'm a jerk. On the other hand, I don't want to be eliminated early since I'll be leaving anyway ... and given my line of work, layoffs are a possibility.

Help!