Hook

If you're going on safari and want to get some really good wildlife shots then you'll need something even longer than a 300, unless you can be sure you'll be able to get really close to the animals. You may become frustrated at missing shots due to the limited range.
The Sigma 50-500 is an excellent lens - although very heavy, and quite expensive. I owned one a while ago and used it extensively for motorsport photography. It's capable of producing some stunning shots in the right hands. In bright outdoor conditions you'll be able be to keep the shutter speeds up high enough to elminate lens shake - this is only a problem in low light. A monopod may be a better idea in these conditions - tripods take a while to get up. I never bothered with either.
Sigma also make a cheaper 170-500 lens which I haven't used so I can't comment on. Canon lenses in this sort of range are seriously expensive!
You're best option is probably to find a good second hand Sigma on ebay for the trip and then resell when you've finished with it - good lenses are very easy to sell on.

Probably a bit late but I got rid of all my Canon L glass and just use a 28 - 135 IS Canon lens. Canon Zoom W/A-Telephoto EF 28-135mm f/3.5-5.6 IS Image Stabilizer USM AF Lens

It is compact, fairly light, covers a reasonable range, has image stabilization, reasonable replacement cost (comparatively) and means I am not changing the lens in adverse conditions. The only other lenses I have thought of taking are a 15mm 2.8 fisheye and a spare 50mm 1.8 (cheap spare).

Lens decision
 

IMHO, when we talk about 35 mm SLR or digital best lenses are:
* Leica, Carl Zeiss and Nikon

Beside talking general brand, the make and the quality of the lens vary the price.

For example there is two different 28-70 mm lenses with Nikkor. One is 28-70/3.5-4.5 and the other one is 28-70/2.8 D IF-ED. How much is the difference between this two lenses? 4 or 5 times. Which one should we buy?

When I read your thread, it seems Sigma lenses fits much more to your needs. It is cheaper and not bad.

I am new to digital, I am more in to film and can't advice which is better. Digital looks as it needs some more time to decide which one is better. I have a 10.2 MP camera at my hand and when I look at the best picture I have taken with it and can't compare with a photo taken with 35 mm film camera of same brand like Nikon though I am not talking about professional studio digital eqipment and backs like Sinar, Leaf or Phase one which are reached to 44 MP.

I think you can never compare a point and shoot camera to a SLR !

Hope this helps

Best decision of lenses for me
 

To my opinion best choice of Nikkor lenses should be as follow: it is better to use a few zoom lenses with smaller difference between the initial and lateral factor instead of a huge 5-500:

35 mm / Digital
17-35/2.8 D IF-ED / 12-24/4 G IF-ED
28-70/2.8 D IF-ED / 17-55/2.8 G IF-ED
80-200/2.8 D IF-ED / ? I don't know if it exist in digital

On my D70s I use a Tamron AF 18/200 mm that I am very happy with. It is the ideal travel lens for me.
I also have a Nikon Nikkor 70/300 mm that I rarely use. I don't want to have all that glass in my topcase and with the Tamron I can do almost everything I want.

I have one of those sigma lenses for my old nikon, they are excellent value for money.

I stopped carrying all that photo equipment about as it impinged too much on my enjoyment of the moment. I now carry a small pocket digital camera. get one with lots of optical zoom, and turn off the digital zoom.