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...
Not strictly correct. Terminology is a little out.
What you have explained is the difference in fill
Power This is:
The higher the rating 750/850+ the better quality the down, and overall lighter because you need less of the good stuff to achieve the same temperature rating.
when comparing down bags you also need to factor in the fill
Weight This is:
The amount of down in the bag usually in grams.
To compare anything there is a good European standard called the EN13537. Whilst many says it has it flaws, it at least allows you to compare bags like for like. This system is becoming adopted by US manufacturers like Marmot and a few others.
Testing is carried out like this:
EN13537 requires a thermal manikin test which produces four temperature results — upper limit, comfort, lower limit and extreme. These temperatures were worked for normal consumers.
The standard measures four temperature ratings:
Upper Limit — the temperature at which a standard man can sleep without excessive perspiration. It is established with the hood and zippers open and with the arms outside of the bag.
Comfort — the temperature at which a standard woman can expect to sleep comfortably in a relaxed position.
Lower Limit — the temperature at which a standard man can sleep for eight hours in a curled position without waking.
Extreme — the minimum temperature at which a standard woman can remain for six hours without risk of death from hypothermia (though frostbite is still possible).
For the purpose of these measurements, a "standard man" is assumed to be 25 years old, with a height of 1.73 m and a weight of 73 kg; a "standard woman" is assumed to be 25 years old, with a height of 1.60 m and a weight of 60 kg.
Using this system you can compare bags like for like. For example, I recently had a Mountain Equipment Titan 850 that was comfort limit rated to -12c. This bag uses 625EU down with a fill weight of 850 grams.
for Norway next month I needed something will a little more punch so started looking around. I found a RAB Ascent 900 at a good price and that is comfort limit to -19c. This bag uses 650EU down with a fill weight of 900 grams.
You can see a fairly substantial gain in temp rating for only 100g more down that is slightly better quality.
I ended up buying a Tundra Pure & Dry -20 which uses (EU) 860+ ethical down and is the best down commercially available. It only packs 800 grams of this but is rated slightly better than the RAB and is around 200 grams lighter overall in weight.
There are a few rating systems out there and I would work to see what you are comfortable with. With my original Titamn 850 I was fine to -9/-10 so happy the comfort limit is fine for me. For Norway I think -20 will be ok and we have extra layers and down clothing for when it reaches -30/-40.
I hope this clears things up a little and not confused it more. I have been going around in circles for the past month trying to buy the right bags for our trip!
G