Tokoloshe has suggested the following, if you are not planning to use the GPS handheld. This is the GPS we use on our trips in Southern Africa:
Garmin Streetpilot 2610 which should come with Mapsource software and Europe City Navigator V6. This GPS can be obtained from many places, but the cheapest we found was from www.aspidshop.com
Aspid are based in Portugal but ship worldwide and there is now no additional duty incurred if buying in Europe (apart from Portugal's normal 19% vat
The most comprehensive Southern African maps we've found are, ironically, available for free from www.tracks4africa.co.za
. T4A also produce a set of "pro" maps for a large part of Africa (except central africa ) for R460, although you can pick and choose from a selection and pay per map. We've just downloaded the set and they look pretty good.
The free maps helped us navigate from Pafuri to Inhambane in October last year although we found some of the waypoints in Mapungubwe to be slightly off route.
If you want autorouting within SA, you need Garmin City Select for South Africa. This will set you back about 300 euros - not had any experience of it, but suspect it will not be of much use outside of urban areas and certainly no use in Botswana.
Alternative GPS is the Garmin Quest which is cheaper than the 2610, but with a smaller display (also primarily for use in a vehicle). For handheld, most of the Garmin range are good and reliable, but I can't comment on mapping software compatibility as we don't use them in South Africa.
Another alternative would be to use a PDA with mapping software (again, don't know if T4A maps will work) - this has the advantage of being relatively large display and car-mountable, but also small enough to tuck into your pocket if you're on the move outside a car. All these options will allow you to upload waypoints to your PC via usb or card. (Streetpilot and Quest mapsets use CF cards for storage)
Hope this helps!