but also remember that whatever temp it is outside isn't necessarily what temp the bag needs to be. it's the full combination of your clothing, etc. so if you're looking to go lighter weight (i'm a lightweight backpacker who has to trim fractions of ounces where possible, since my base weight for my pack...everything other than water and food...is 10 pounds), you don't need quite such a low rating. is this going to be for actual backpacking? or is it going to be for car camping? if car camping, then you obviously don't have to worry about weight or anything at all.
also, some brands are pretty true to their rating, some aren't. 20 degrees from patagonia can be totally different than 20 degrees from lafuma.
my main bag is a 40 degree adrenaline from golite.com. i don't enjoy winter camping, so the 40 degree is sufficient for 99% of what i like to do. i also have a 20 degree cat's cradle from northface, but it's SO much heavier that i avoid using it at all costs (unless i'm car camping so that the weight doesn't matter). all of these types of bags come in "regular" or "long," so length shouldn't be an issue.