I haven't used one, but try Travel & Leisure online as a resource - they regularly do articles about good travel agents for all budgets, and most of their content is available online. It seems that Africa is a popular place to use their services since it is so large and diverse, and not as "touristy" in most locations.
Well they make their money off booking things for you, so I don't see them being terribly helpful. I've always liked internet forums (big surprise!) to ask about neighborhoods and activities and such. Sounds like a lot of fun!
I recommend contacting Rhino Safaris for Africa travel www.rhinoafrica.com/. They work with all budgets and have really good connections with hotels/vendors that can save you money, so you don't really end up spending any extra by having them do the work for you. They're VERY good and I haven't met anyone who wasn't thrilled with their work.
I would suggest getting a travel book for the country/area that you are looking at travelling to. We usually use Lonely Planet. It will give you suggestions on where to stay, what to do, and where to eat.