You don't need to wait after using Roundup. But first- you want a warm sunny, not windy day. I would try to work some sand, peat and manure into that bed.

Another thing, you can use Preen to get rid of weeds. You can't plant seeds into the bed for several months, but plants are ok.

I think I am finally retaining the info from my Master Gardener's classes.

Anytime you till, you'll 'sow' weed seeds and possibly stimulate the plants already growing by chopping the roots into tiny pieces, each of which is likely to grow a new plant. In an area that small, it should be pretty easy to pull them up. Mulch when finished to prevent the return of most of them. If you're willing to dig up the bed, why not just pull/dig up the weeds? Lots less work.

If you're not in a huge hurry, make a lasagna bed out of it. Lay down newspaper (8-12 sheets thick) to kill the weeds, then make the equivalent of a huge compost pile on top of the paper. You can do the latter in layers (hence the "lasagna" name) or just mix it up. Check out the soil forum for more ideas on what to put on there.

You can still plant right away - just dig through the newspaper, and make a pocket of soil to plant in.

Here is a link that might be useful: Soil forum is down to earth

Forgot to ruminate... they 'teach' Round-up and Preen in master gardener classes? Aren't there extensive instructions (and warnings) on the package? Do the chemical companies sponsor the classes? Applying chemicals is not a gardening skill, imo. Anybody can read and follow directions.