Yeah, roundup kills EVERYTHING, its like the napalm of yard products. And while its some seriously harsh chemicals, I wouldn't really hesitate to use it in a yard. I'd never use it in a spot where I'd eventually be growing a garden or something, but it's not like you or your pets are going to be eating grass on a regular basis, so I wouldn't worry about it.
Most standard varieties of roundup would be safe to plant grass 14-30 days after roundup application(I'd probably wait for the 30 day mark just to be safe, but that may be overkill. The only exception being The Roundup Extended Control Weed & Grass killer, that shit lasts for MONTHS, I think they say not to plant anything for 3-4 months after using that, so basically don't use that, lol.
Id probably do something like:
Step 1 - Apply roundup to yard(probably buy the bulk product that you mix with water and use your own sprayer, using the little Roundup built-in sprayers will take FOREVER)
Step 2 - Wait 5-7 days, lots of stuff will be brown and dying by that point, but you'll probably still have green spots that you missed, Roundup the yard again, especially heavy on those green spots
Step 3 - Wait 2+ weeks, and rent an aerator and seed the lawn. You don't have to specifically aerate really, you just want to do something to the yard to where you arent throwing down seed on flat, compacted topsoil. You need to break up the dirt a bit to give the seed the ability to take root easier, and it also prevents as much runoff of seed from raining/watering. If I were doing a whole yard, I'd probably aerate as thats the quickest/easiest route. You could also till the yard, but that would be more work and take longer(but that would probably yield the best results, as you'd be loosening the dirt in the entire yard, not just in spots like an aerator), or you could also manually rake/hoe the dirt to break it up but that would be a HUGE bitch. I only go the manual route if I'm spot seeding a small bare spot or something.