It is perhaps a bit surprising that both Great Zimbabwe AND Khami ruins are registered as separate WHS. They are however from different, albeit related, cultures. Historically the former preceded the latter. My personal view is that, unless you are either a “WHS nut” or particularly interested in pre-colonial African ruins/history then you should concentrate your efforts on getting to Great Zimbabwe and give Khami a miss.
If you are in Bulawayo then Khami is only 22kms away but you will need your own transport or have to pay for a tour and the entrance fees for foreigners reflect a certain lack of realism as to the value of what is “on offer”.
Khami is a court complex of the Torwa state from around the 15th century. Like all “Zimbabwes” the palace is built on a hill and a number of stone walls from the structures partially remain. I didn’t personally didn’t find it a particularly worthwhile place to visit when we did so in 1997.
A few miles away however the ruins of Naletale were in a totally different class. Not only was there no entrance hassle, the site offered complete peace and the most wonderful patterned brick/stonework – far better than you will see at either Great Zimbabwe or Khami. There are apparently 5 types of pattern and Naletale has fine examples of all of them. From a distance it looks like a tapestry in stone. Look at a photo at http://www.geocities.com/thetropics/island/6697/africa9899/pic10.html