Alan, I would allow at least an hour to drive to Boulders from central Cape Town. Also, there are roadworks from Muizenberg south on the M4 along False Bay. A good alternative route is to take Boyes Drive, which parallels the area of the roadworks.
I'd suggest that you go to the AA office in the Picbel Parkade, which is very near the Cape Town Tourist Info Centre and quite near you in Waterkant — just walk a few blocks east along Waterkant or Strand Street, and pick up some maps. If you're a member of AAA in the US, you have reciprocal membership in AA and the maps are free (I stock up almost every year

).
There are not a lot of alternative routes south. I'm not sure whether Chapman's Peak Drive has reopened, as that would be a scenic alternate route — that would be M6 south out of Hout Bay and then across the pennisula to Glencairn just north of Simonstown.
Another alternate route would be the M64 Ou Kaapseweg — take a right from the southern end of the M3, then left onto OKW. It's a very twisty but lovely route, so you need to take your time, and it eventually joins up with the M6. The AA's Cape Town Metropolitan Area and Pennisula map will make all this clear, it's what I'm using now because while I know all the alternatie routes, having driven them often, I don't always know the route numbers!!
Enjoy your time in the Cape, my home from home!

@anita: Best you not rely on your satnav, but get yourself some AA maps