I'm planning on hiking about half of the AT Northbound (starting at Springer Mountain) at the start of May 2013. What sort of temperature ranges can I expect to encounter, and what should I look for in a sleeping bag to best suit these? I will be using a tent pretty much every night.

My main consideration has to be weight, so the lighter the better. However, I would like to be comfortable at least some of the time but appreciate that lower temperature comfort limits will add weight.

As well as this, I'm conscious that lightweight + warmth = high cost, and bearing in mind I will be on a fairly strict budget I'm reluctant to just drop £300+ on a bag, so are there any specific manufactures that offer the best value for money? One option I'm considering is a Mountain Hardwear Lamina 32 combined with a liner for it gets colder. I can get one of these for about £65, would it be any good? Or not warm enough?

I am from England and know nothing about camping / hiking in North America so any other advice, however general, would be greatly appreciated.