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How much topsoil do I need for planting a flower garden?

I could really use some good gardening advice on this one! Okay, here's the situation: We dug up our entire front yard to put in an entrance to our basement. What was left there was a mixture of gravel and compacted clay-like earth. (the entire yard was interlocked before it was dug up) After sitting for a couple years while we renovated inside and spent copious amounts of money doing so :( we have finally got the yard interlocked and left a large portion for a flower garden. The workers brought in a dump of topsoil, but it's only between 3-4 inches deep. Beneath that is the hard, compacted clay stuff. So my question is how deep does the topsoil need to be to have a healthy flower garden? I would assume that water drainage could be a big problem. So do we have to dig it all up and bring in another load to make the topsoil deeper? Or can we just till it all together to about a foot deep so that the clay stuff gets mixed in with the soil? As you can see, we're novice gardeners. Please help us out!

Public Comments

1. a minimum of 6"
2. 4" of top soil and mix well with 2" peat moss and the existing soil. If you have a rototiller that would be good otherwise use a shovel and turn it and mix well. If you can buy a few hundred worms to put under the new soil, it will help to soften the clay below.
3. If you could get a load of compost rather than more top soil, about the same amount as the top soil, and then till it all together and get it to 6" or about that, it would do best. The compost will add nutrients and drainage which plain top soil won't. I see above someone recommended peat moss, but it is not normally recommended for gardens. Compost is the choice most gardener's use with the clay soil. (We have clay soil and the community garden I work with uses 1/2 top soil and 1/2 compost and the garden plots are big veggie producers.)
4. Here's what I've always found for my garden..... However much you think you need, get twice that amount. I always underestimate how much will actually be required and have to go back 2 or 3 times for more soil! Plus, often it will settle after watering and sink down so you'll need more to build the bed back up. If you're growing veggies that require depth (beets,carrots, onion...) then you'll need more soil to create the depth.
5. When we moved to our new house several years ago, the soil had a great deal of clay. To make flower beds, we also use bulk compost mixed with sand and some top soil to make flower beds. We also added one other thing. That was gypsum pellets. The gypsum helps break the clay into smaller particles and incorporate them in with the other material.