I am by no means an expert, but I'll be more than happy to share what I've come to understand from reading and listening (until the real experts chime in!)
I recently attended a seminar given by Texas Agrilife Extension Service; and they said that, at least in
Dallas, Bermuda and St. Augustine are the only lawns that work well.
I'm on the last leg of battling weeds, so I empathize. My
novice opinion would be to remove the top layer (and the weed seeds), and put in corn gluten meal and till in compost. But on the Dirt Doctor site there is great information on how to prep a new lawn
http://www.dirtdoctor.com/organic/garde ... n/id/1822/ Please avoid the chemical herbicides. A neighbor had their turf removed, treated with a chemical herbicide, and new sod laid; and I was so sad for them to see all the weeds in their lawn this past year. The corn gluten meal is a pre-emergent weed killer, when applied at the right times. Good lawn care practices (mowing, watering, fertilizing, and weeding) will help take care of any weeds that may pop up - there is so much good information on this site and forum to address the details. Things like diatomaceous earth and beneficial nematodes can take care of any pest problems - like chinch bugs, and organic anti-fungals like corn meal and milk can take care of the fungus that may have walloped your yard this spring (more details are on this forum).
So, hopefully, this is a little more info. that may be of help, in addition to what Char said!