After you've sprayed with roundup and have the area planted, I would use pre-emergent on it to help keep the weeds out. I've been using it for years on some of my sedum and semp beds with pretty good results.
The deciduous sedums, which Purple Emperor is, are especially hardy since they go dormant in the winter.
I don't know exactly how hardy sedum alba is, but mine actually grew this winter when it was below freezing, so I'm thinking that one might be a good one to try. I'll try and take a picture of it.
