Gardening Question

As some of you may recall, last summer I had some friends come help me redo my backyard. We killed all the weeds, rototilled the soil, planted new grass, and lo and behold, I had an actual flat and grassy backyard that I could mow. Then my grass suffered the devastation of leaf rot:

http://logisticslad.livejournal.com/62353.html

Now I have a backyard that consists of patches of healthy grass, patches of bare fertile topsoil, and lots of different types of weeds. I went to a garden store and they recommended that I reseed with new grass, wait about 4 weeks, and then treat with a soil treatment like Turf Builder 2, which is supposed to prevent new weeds and kill existing weeds, but not grass. My friends D & E advised me to do the reverse. Kill the existing weeds with a chemical spray that is designed to preserve the grass (any recommends?), wait a week or so and then reseed with new grass. Apparently, weed killers and germination inhibitors are hard on young grass, so that whatever I do, I shouldn't treat the new grass until it's been above ground for about a month. Spring and Fall are the best times for planting grass seed. So which suggestion makes the most sense?