Raised beds, how high or does it matter?

I am planning to make some raised beds for my first attempt next year at vegetables!  I have some old planks and posts for the corners, but don't know whether to make the beds one or two planks high?   Or does it not really matter?  Also as the reason for making raised beds is that my soil is impossibly stoney what is the best thing to fill the beds with?  I was thinking of a mix of destoned soil and compost as far as I can and then maybe I might have to buy something to top up with but not sure what?  

All advice gratefully received.

Our raised beds are 150mm high but we are also digging out the paths around them so they will end up being about 200-250mm high which seems just about right for us as it takes the strain out of bending too much.

Our soil is very claggy so we are digging in a mixture of hay and guinea pig poo as my wife has about 50 of the things so there's a surplus and it's braking the clay up really well.We are also adding some well rotted compost and rotten leaf mold and they soon fill up with all this added.

just build them higher than the bloke has his in the next plot.

build them as high as you like/can. you're the one who'll be bending to tend to them so it's a case of how much do you want to bend/how much soil do you have to put in them. the deeper the growing medium then potentially the better your crop.