Temp rating and fill weight are two very different things. Temperature ratings are created by the manufacturer as an estimate as to whether you will A) be warm at the specified temperature or B) survive at that temperature. There is no standardization between companies, and it's really just an estimate anyway as everyone sleeps differently.
The fill weight is in reference to the weight of the down that keeps you warm. The higher the number (ie 800 or 900), the less down you need to maintain the same R value. The lower the number (400-600), the more down you'll need for the same R value. Because of this, the higher fill count bags will pack smaller, weigh less, and have less loft (puffiness) than a lower fill-weight bag. Loft is usually measured by height...from the floor to the top of the sleeping bag when it's laid out and fully fluffed. Looking at one of the comparison charts from either Western Mountaineering or Feathered Friends should give you an approximate idea as to how much down is equivalent to an intended temperature rating with the specific fill value.
Correct me if I'm wrong, but most of the cheaper bags are synthetic, which is great if weight/space isn't much of an issue (car camping, canoeing, etc) and have the added bonus of still being somewhat effective at retaining warmth even when wet. Down is useless when wet. The whole loft/fill weight thing doesn't apply to synthetic bags as you're comparing different insulating materials.
I hope that helps clarify things...
