Good replies so far, just a couple additions.
Round-Up (generic = glyphosate) is a post-emergent. As beenthere said, kills all growing weeds. If you go with something that's mixed with something else, the something else could be a pre-emergent, thus will kill your ready-to-plant-grass SEED (if I understand you correctly).
As I recall, Round-Up wasn't designed for broadleaf weeds and some folks have a lot of broadleafs and it hasn't worked well for them. I believe the label says it works on some. For hard to kill broadleafs, I spray them a little more heavily and they die.
Some glyphosates have the surfacant built in, but some might not. You can, and should, use a surfacant .. dish detergent works.
As far as subsequent dew and rain goes, i.e., rain-fastness, that is something to check into for each different brand ... seems it's usually buried in the encyclopedia that comes with the chemical. It seems to vary from a couple of hours to a couple of days.
Cheers!