Why not join in a tour? iRide Africa is a local company with guides that know all the trails. Mtb places are rarely signposted and not well documented. Jonkershoek and delvera winefarm have some good trails in Stellenbosch. Cape town's best riding is on devils peak and tokai but there are many more trails that are harder to give a location for.

Check out http://maps.crank.co.za for trails in your area.

I don't live in CT but have ridden a few great trails. Tokai and Jonkershoek are amongst the best but there are many others.

http://www.mtbroutes.co.za has quite a few listed.

Google earth also lists som. Tokai, Silvermine, Jonkershoek and Cedarberg are highly rated. Also don't underestimate the 1000's of km's of farm roads between CPT and Namibia!