Anyway, 1) No matter what the height of the fence, I would always go at least 2 feet deep. Above a 6' fence, I would make the ratio 3/4 above ground for each post.
2) The distance between posts has a couple of factors: What are you fencing? If it is purely decorative, strength is not a factor. If you are holding cows, you need them a lot closer together. How long are your rails? How tall is the fence going to be? A good ratio for post spacing is 3/4. In other words, a 6' fence should have a little less than 8' between the posts. a 3' fence should have something closer to 4' between posts, otherwise it looks out of proportion. (to me, anyway)
I always backfil with concrete, but that's because I don't want to have to redo the project in 4 years. Rent a one man auger (it comes on wheels, with the motor on the wheel end and you and the auger on the handle end, much better than the two man auger with the handle and motor above the blade with no wheels) The auger makes all the difference in the world, unless you have a 15 year old kid in the house who thinks he wants to play football this fall, then buy a post hole digger.
A concrete mixer is a great rental too. Most rental shops have mixers you should be able to get home in your car, or most will deliver for a small fee.
Plan on 2.5 60# sacks of premix concrete for each 2' hole. 60# sacks run about $1.50 each last I looked. Mixed concret should be just a little harder to pour than stiff oatmeal. You don't want it to be watery.
For more information, hit ctrl-f and enter fence in the find box.