An important thing when you consider a tele is that for the 400D you have to multiply with 1,6 (because of the sensor) to get the equivalent of the same lens on a film SLR. Now, to get a sharp pic with a lens, you have to shoot at a speed that is faster than your lens "length".
Meaning that to get sharp pics with a 200mm, you must roughly shoot at 1/250 or faster.
Unless you have a tripod (and use it) or a lens with image-stabilisation (costs more than a leg).

The advantage of the 400 over the S5 is that at higher sensitivities you can still shoot without too much noise. The advantage of the S5 is that the lens has image-stabilisation.

My very personal opinion is that it's very difficult to get acceptably sharp pics when you shoot handheld with a lens of more than some 135mm (meaning 216mm in film speak). Also (even more personal) tele's tend to flatten things, and if you shoot things far away, you get atmospheric effects like haze etc. So a really long tele is for specialized work. Which translates as money!

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