Decide what temperatures you expect on the coldest days and go from there. Many people (and I have been one of them!) haul around more bag than they need. Add to your bags warmth with the clothes you wear (make those clothes do double duty and plan to sleep in them). Evaluate what kind of sleeper you are, I sleep colder than almost anybody, and I do need more bag than most (I am tall and built like a stick). Recognize also that a good nights sleep can make or break a longer trip, and a sleeping bag may be your ultimate survival tool if you end up facing the unexpected.
Consider a quilt! This is a whole different idea, based on the fact that you will have a pad under you anyway and the part of a sleeping bad under you is just getting squashed flat and would be on top of you and keeping you warm if you were using a quilt. The idea makes sense, and I have gone the quilt route entirely with no regrets.
Once you have fancy and expensive down gear, take care of it! Apart from just not throwing it in the mud on a trip, learn how to properly wash and dry your down gear.
You may need several bags. A super light summer bag, and a warmer bag if you do winter or late fall trips. You cannot have too many sleeping bags. A versatile option is a liner bag (Mountain Hardware makes what they call "upgrade bags" that zip into their regular bags). A liner/upgrade can be used by itself in really mild weather. The main bag can be used alone. In really cold weather, take them both! The only drawback of these upgrade bags is the extra zipper hardware adds weight.
Read these notes on choosing a summer sleeping bag.
I am a very cold sleeper. I will be chilled while others are uncomfortably hot. Here is a data point from September of 2009. I slept in the bed of my truck at 8550 feet in light clothes in my 35 degree rated "Big Agnes" bag on a thermarest camp-rest pad. I was right at the limit of being just warm enough. Temperatures were steady at 54 degrees, with a dip to 52 degrees during the hour or two before sunrise when clouds gave way to clear skies. Wind was fairly steady at 16 miles per hour, but the truck bed gave some shielding from the wind (but I could feel the wind buffeting the bag through the night). The wind certainly robbed some warmth, I would have been cosier in still air, but the Big Agness has a pretty tight shell. I have the temperature and wind data because this was in the parking lot of the observatory where I work and weather station sensors were less than 10 feet from the truck.
This confirms a rule of thumb for my metabolism that I should add 20 degrees to the rating of any bag that I use. A 35 degree bag just works for me at 55 degrees. For me a 50 degree bag would be just the thing at 70 degrees. My 15 degree rated Marmot, will work for me at 35 degrees. I am not able to "tough it out" below this threshold; I will be miserable and will not get a good nights sleep. Everyone should work out these sort of guidelines for themselves.
And finally there is the question of down versus polyester stuffing. Down is by far the best thing there is, as long as you keep it dry! Down costs more up front, but will be cheaper over the long haul and will perform better. A wet down bag is useless and almost impossible to dry in the field. Polyester does pretty good even when wet or damp, but is more bulky, and after a couple of years of steady use only fluffs up half as much as it used to. I have heard it said that if you let your down bag get wet, then you are to blame. The most common cause of wet down is condensation inside a tent (assuming you stuff the bag in a waterproof container in your pack and have adequate shelter). There is always the unexpected.
Down will last almost forever if it is not abused. Most down (and down bags) are wrecked by improper washing. The best thing is just to avoid washing them. Consider some kind of bag liner that can be removed and washed. If your bag does get wet, get it dryed out as promptly as possible (and don't wreck it in the process). A bag stored wet will mildew and be ruined. Store any bag, but especially a down bag fluffed up in some kind of protective storage bag that allows ventilation.
Western Mountaineering makes some of the very best down bags. If I didn't already own a marmot pinnacle long (an 800 fill 15 degree rated down bag), I would be scheming to buy one of their bags, perhaps the 15 degree rated Apache with 21 ounces of down (34 ounces). My marmot (in the long version) is 52 ounces, with 24 ounces of down does weight about a pound more, but is a very nice bag, and it is hard to justify spending the $400 or so to get the WM bag, much as people rave about them.
Feathered Friends makes sleeping bags that are equally as legendary as western mountaineering. Their 10 degree rated "raven" has 23.4 ounces of down and weighs 42 ounces. They make the amazing "rock wren" bag that is wearable (has a drawstring at the bottom that your feet can pop out of!). The warmer winter wren version in long weighs 36 ounces, and is rated to 25 degrees, $350.
One option that I am part way into investigating is sewing my own sythetic fill quilt. To this end, I purchased a Ray-Way quilt kit, but have yet to tackle the sewing. I purchased it with the "alpine upgrade", which provides two layers of 0.9 inch insulation rather than two layers of 0.75 inch insulation. This quilt without the alpine upgrade weighs 25 ounces.
Another highly regarded option is to purchase a down quilt from Jacks R Better. Take a look at their quilt comparison chart. Given that I am always very cold, I ordered a Rocky Mountain Sniveler (long) with 1 ounce of overfill. It should weight 29 ounces with 18 ounces of 800+ down fill. A nice feature is that it can be worn as a "down poncho" allowing some warm relaxing around camp. They also plan a tapered model they will call the High Sierra Sniveller, but I couldn't wait (the Rocky Mountain is 48 inches wide and rectangular, the High Sierra will be 52 inches at the shoulders, making it somewhat better for a ground sleeper rather than a hammock user (many of the Jacks R Better products have hammock campers in mind).
Here are some articles (I apologize that these are by "subscription only" at the backpacking light website, but that's the way it is.) An unconventional sleep system is anything that is not a plain old sleeping bag. Take note of the "wearable" sleep systems, in particular the "rock wren" from feathered friends.
Tom's hiking pages / tom@mmto.org