I would definitely recommend either the 400D or the new 450D when that becomes available as the spec on that is superb though the former is a cracking camera. Now that the 450D has been announced, you should have no trouble finding a 400D with the to kit lenses for within the upper range of your budget and I think you'd be absolutely delighted with the results you'll be able to obtain.
I would normally have recommended going up to a 30D or a 40D but the 40D is a bit over your budget, especially when you add lenses on, and they are considerably bigger than the 400D whch will be better for you to carry on your trip.
I have both a 40D and an S5 IS (well, my wife has the latter
) and as great a camera as it most certainly is, I do not believe you will get the great shots that a dSLR will give you, especially at the spped you want. You are not going to get 3-4fps from it, that's for sure! As you intend doing some long exposure shots, you really should be thinking dSLR only. The levels of noise on a compact will quickly become very noticeable whereas the dSLR copes with that far better. For your sports shots, there is a small but noticeable lag on the compact which isn't there on a dSLR, another worthwhile consideration.
Look around and you'll find the 400D at an excellent price and I am certain you'll be coming back from Africa with gigabytes of photos to show us
Cheers,
Rob