If there's only 6-7 inches of soil there then one of the biggest problems is going to be depth.
You're going to need to put in beds of 10-12" deep, which means that somehow you're going to have to fill them with soil.
You can solve part of the problem by putting in 2ft wide paths and lowering the path by say 5" and putting that soil into the bed, but you're still going to have to find an awful lot of soil from somewhere. If you've got any free sources of local manure then you could rob peter to pay paul, construct 6 beds but put all the soil from 4 of them and all the paths and fill 2, then fill the other 4 with manure and leave until next year.
Raised beds are great where you have major soil problems, like none or very wet heavy soil as they dry quicker and warm quicker. They also give you a quick psychological boost as it doesn't take long to turn/dig/weed/plant an 8'x4' bed but the effect is instantaneous even if you got another 8 to do.
The down side is the opposite because they drain quicker they suffer more in drought. Unless you adopt "grid gardening" or "square foot" methods you can waste more space, but by adopting these you can actually produce more but at a price in terms of maintenance - you need to feed the soil and maintain its structure.
Hope this helps.
