ecc,
I would take a good look at a
Tarptent . These shelters are incredibly light, very sturdy, very weatherproof, provide adequate ventilation, and are bugproof. As an example, you can get the Cloudburst (2-man) with a sewn in floor, providing 39 square feet of floor space, and still weigh in at 2 lbs, 6.5 oz. That weight includes stakes and poles. The Squall is an even lighter 2-man shelter at 1 lb, 14.5 oz. If you're looking for even more space, Tarptent is now making a 3-man shelter called the Rainshadow that weighs in at 2 lb, 9 oz. with a sewn-in floor, poles, stakes, etc.
If you poke around the website, a lot of the questions you have will probably be answered. I have personally never used a Tarptent, but as soon as I can sell some of my other tents, that's what I'll be buying. I have spoken at length with two other backpackers who have used these shelters extensively, and they both rave about them. The shelters are not free standing, but that rarely seems to be a problem.
I know there are some other fans of the Tarptents over on the AMC boards, so you might get some good feedback over there as well.
John