Two weeks minimum; four weeks maximum. The two weeks gives time enough for the machinery at your current employer to crank up for replacing you (or trying to get you to stay!). It's the professional thing to do. Positions that carry a higher degree of responsibility require a little more notice, but no more than four weeks because once you give notice, you're a lame duck. You won't be kept in the loop. You won't be asked to attend meetings. There'll be no raises, of course, or promotions to someone on his/her way out. And they may want you to train the new person if they hire before you leave. That's a huge drag and shouldn't be your responsibility.
Here's what I recommend: I think you should get a break between jobs, too, because you won't get another vacation for a while. Give three (or two) weeks notice, but tell your employer you'll be ready to start in four (or three) weeks. that'll give you a week off.
Not giving someone who's laid off notice is optional unfortunately. I think it's pretty crummy to not at least hint that there's gonna be cutbacks and that it may affect you. So, not a very nice place to work. Congrats on getting out. Still, be better than them and do the professional thing, but do not feel guilty about leaving. Most of your bosses would be out of there SO fast for more money and/or better benefits. Don't let them make you feel like a creep for wanting to improve your status.
Good luck and congrats on the new job!