I've been looking at the Canon EF 400mm f/5.6 USM and the Canon EF 100-400mmf/4.5-5.6 IS USM as well as the Cannon 400mm f/2.8LIS USM and EF400mm f/4 DO IS USM.
Personally, I think the best 'bang for the buck' is the 100-400mm f/4.5-5.6 IS USM. It is sharp enough, has the flexibility of a zoom, can take 1.4x/2.0x teleconverters, and has L glass construction quality. The downsides to this lens are the older (version 1.5) of IS and the push/pull zoom. The IS is still very useful and the zoom just takes some getting used to. It is also light enough to handhold (3 pounds) and only costs ~$1300 USD.
The 400mm f/2.8 L is indeed a sharper lens and handles TCs better for AF. However, it is ~$6200 USD and weighs in at 11.7 pounds. A serious bit of glass. No IS, but one would have to be a circus strongman to try to handhold this puppy.
The 400mm f/4 DO IS USM weighs in at a mere 4.3 pounds but costs ~$5300 USD. Most reviewers think Canon still has a long way to go with their DO technology, although, I believe I have seen a few shots on this site by Mr. Reichmann using a DO lens. You probably could handhold this lens with the IS, but I don't think I'd want to shell out the bucks for this particular offering.
Since you state that you are an amateur (like myself), I'd suggest going for the 100-400mm zoom first. You can walk around with it, have the flexibility of a zoom, not break your back lugging it from place to place, and get very acceptable image quality. You can add a TC (I use a Kenko 1.4x Pro) without losing much. It also won't break the bank like a $5k+ lens might. With the savings, you can get some very nice additional gear (tripod, ballhead, TC, remote, etc.).