My wife and I were looking at the Kelty cosmic down last weekend as a choice for her (she gets very cold). It helps to see a bag in the store. I was looking at bags online, and when I went down to the REI in town the bag was really bulky (even for down). Personally, I'm in limbo as to what to do about a sleeping bag due to price and the fact that I always seem to hike in wet weather (caution for down bags). I have a 20 year old North Face cheapie that only recently has started to feel cold. I picked up some silk long underwear from Sierra Traders for cheap. My wife's bag is only rated at 20 F, and she has needed a liner to keep her warm. Last year she used a flannel liner, silk long johns, and she was fine in 30 F weather. This year, instead of spending the money on new bags, we picked up a thermolite liner (much lighter) for her and will find out if it is useful.

If you're extremely new to backpacking, the first question is "Am I going to keep this up for the rest of my life?" The answer to that question tells you how much to spend on gear. I've been upgrading my gear whenever I can afford it, but I certainly did not go out and buy expensive gear until I knew that backpacking was like water (-- couldn't live without it).

Get a good pad- thermarest or otherwise. Insulation matters.