In reply to zarathustra:
Save up! Alpkit, hyperlight, zpacks.. or go for a well known brand stocked in mainstream stores.
Make sure you look at the comfort rating (comfy sleep at this temp) and not the extreme rating (the temp at which you probably won't die but may not sleep) and bear in mind if you are fat/tall/short then bags will need to fit you snugly to be effective.
Check features like zip and shoulder bafflers, foot boxes, waterproofness, etc, which side do you get out? Which side do you sleep on? Bag choice is very personal.