I think a .30-06 or .300 that you shoot well with tough 180-grain bullets are what I would recommend. The PH we hunted with this year recommended .300 Win Mag with the same Barnes Vortex 180 TTSX that NTO recommended in this thread. These days I mainly hunt with custom FN Mausers in Brown or High Tech stocks or a Blaser R8, which are a great set up for the money if you can find the models witout a detachable trigger for around $2k. A M70, M77, Tikka and a lot of other rifles work also.

A few things I have learned about rifles and ammo on my African trips:

-Bring a rifle that you shoot well and shoot it enough to work out feeding issues (intentionally cycle and practice shooting quickly from the shoulder to see if you can create a feeding issue).
-Know where said rifle shoots with both a cold and warm barrel. I had a rifle I took that would clover leaf, but I hadn't shot it enough to realize that on a cold barrel it was 3-4" high and as much left every time. This plus having the gun sighted in 2.5" high at 100 made for a long day of chasing a badly hit Oryx on a 200 yard shot.
-I have hunted Limpopo and Namibia. The PH's there have recommended a 100 yard zero or maybe 1" high, which seems to work well in that country.
- I used to like light rifles but am realizing I shoot better with more barrel weight, especially off of sticks. I am trending to heavier rifles. Most of my trips we have probably walked five miles per day or so in sandy, rocky country. I am good with carrying a 9-pound rifle on five-mile hikes, especially if I have a sling.
-Order some shooting sticks from the place out of Ohio that makes African-style sticks and take them out and shoot on 5-10 occasions before the hunt at ranges from 100-300 yards. Study shot placement and get targets of African animals and use those for practice.
-PHs will often emphasize heavy, tough bullets such as Barnes. Listen to them. I think the hides of many African animals are tougher than NA equivalents and that muscle and bone structure may be as well. My father, myself and daughters, and friends that have hunted with us have caught a much higher percentage of tough bullets like TSXs than we have had go through. As an example this year, I had a 180 TTSX not penetrate to the lungs on a slight quartering shot on an Oryx and had the same bullet on a Blesbock (about the size of a Whitetail) not go through on a broadside shot.

Have fun in Africa.