Yes, provided you get the maps for it. Garmin is a big GPS brand in SA, so that should not be a problem.

In addition Google is investing heavily into their maps of South Africa with street view support, loads of updates, etc... so that will make a great addition or fallback.

By all means you will. Make sure you download the latest Southern African regional software onto your Garmin and you will be able to travel anywhere in South Africa, accurately. In addition the following guys provide specialised maps of Cape Town and surrounds (http://www.gpstravelmaps.com/capetown.php). Also visit Garmin SA (http://www.garmin.co.za/) for up to date info and technical know how.

If you're renting a car, all the rental car agencies offer GPS devices with South African maps for an extra fee.