What environment are you going to be using it in? There are some great technical materials used to make really good, lightweight bags these days, but there are some things to consider. A down bag can pack up really small, be really light and super toasty, but its useless if it gets wet. Some of the different poly fill bags don't pack up as small, and may be a little heavier, but have some insulating value even when wet.
As far as rating goes, you want one rated about 10-15 degrees cooler than you will be using the bag in. A zero degree bag doesn't necessarily mean you are going to be warm at zero, just that you won't get hypothermia.
What dats82 said about stuffing it is correct, in fact, the bag should come with a larger storage bag that allows it some room to breathe when you aren't using it.
Also, consider your whole sleep system. A good pad underneath you not only keeps you more comfortable, it also gives you another layer of insulation between you and that great heat sink known as the ground.
As you are in Murfreesboro, it's probably worth making the drive up to Nashville to check a couple of places like REI that will let you pull the bags down off the shelf and try them out. You don't want to invest in a $200 mummy bag just to find that you're claustrophobic.
Have fun. Be safe.