Jennifer, don't forget that when you get the soil, it will be all fluffed up from being loaded into the delivery truck, and then unloaded at your house again. So if you want your bed to be a foot deep, better pile the soil 18in. or so to allow for settling. You'd be disappointed in the fall when you're ready to plant and find your soil is only 6in. deep.
We used to buy compost at the landfill, and they called a 'yard' of compost one scoop from their front end loader. That amount filled the bed of my son's small Toyota pickup pretty full. That's my 'visual' of a yard of soil if that helps at all.
I think killing the grass with Roundup, then tilling it in and putting the new soil on top is going to work fine for your bed.
