Try Weather Underground
 

You might want to check the Weather Underground for temperatures where you are planning to travel. Some places have an almanac so that you can see what the average temperatures are at the time of year you are planning to go there. Here is the link to Namibia:

http://www.wunderground.com/cgi-bin/...?query=namibia

Hmmmm, 27C for a high and 15C for a low tomorrow in Windhoek...

Calgary: +1 for a high and -7 for a low and we just had a dump of snow. Where would I rather be? :eek2:

www.wunderground.com

Oh yeah, down bag and cotton liner for us. We tried a super light +10 bag (hey, the next trip we are travelling lighter!) on a summer trip from Calgary to Vancouver and nearly froze even wearing fleece clothing. We've used the down/cotton liner combo in everything from +40 to -10 without trouble. Everyone always raves about the silk liners but I've found that it catches on dry skin whereas the cotton doesn't.

never been to africa but........
 

its recommended by the "experts" that you get a bag rated 1 season higher than the conditions your using it in, but thats no use when you cant be sure what those conditions are going to be!
i wont comment on what sort of bag you should have because thats a completely personal preference kind of thing, i cant stand mummy bags for instance. but in my experience its easier to cool a hot bag by opening the zip than it is to warm a cool bag with extra layers.
i use a vango synthetic 4season extra large thing, quite cheap, not a great brand or special features or anything, but its superbly comfy, big, warm, washes well and its well worth the huge packed size (it packs bigger than my 3man tent!) wouldnt go without it.

Snugpak
 

I personally really don't think Snugpak can be beaten for quality and warmth.

Military themed bags, which are much better than the majority of civilian bags that I've used. Very hard wearing and a great range, from their 'jungle' bag - very light (size of my fist when in the compression sack - but only rated to +2.) At the other end of the scale you have arctic bags down to -20, that still pack quite small. I've used my Jungle bag down to about zero, without freezing solid, though it was a bit chilly. It also has a very useful mosquito net built into the hood.

A sleeping mat is invaluable, I just use a cut off cheapo foam one, I always end up puncturing thermarests, and then they are less than useful.

Joel