Serious Wildlife lens question

I'm very fond of wildlife photography. Right now I use a 300D and a 100-400 5.6 for this purpose. As wildlife tends to be in difficult/dark light circumstances I often find myself limited with shutterspeed (1/125 at 5.6 or lower with iso 400 or 800)
The 100-400 brings satisfying results, especially with regard to the flexibility of the zoom. In low light the number of keepers is small, but satisfying.

In order to get more satifying results I'm thinking of buying a faster lens.
Would appreciate your advice!

what are you shooting primarily. probably the best birding lens out there is the 500 F4L IS.

but if you are shooting larger wildlife, or dont mind putting on a TC, you might look at the 300 2.8.

if thats too much cash, you might also look at the sigma 120-300 2.8 and a TC.

Most of the time I'm taking photo's of larger wildlife but birds is also on my to do list
see link
http://www.photo.net/p​hotos/Timo%20de%20Krui​f

so what MM do you find yourself at most? if you feel you need a zoom, but want more light, youll probably want to look at that 120-300 2.8. of course its a heavy monster, and youll need a good monopod at least.

The only complaints you'll hear about the 500 4. are price & size.

zeiss 1700mm f4

If you can afford it then the 300mm f/2.8 will get your shutter speeds up. As a fellow 300D owner I'd also recommend a low-cost trade up to either a 350D or a 20D. They're much better at ISO 400/800 than the 300D.

No matter what you choose, your getting into the very expensive lenses $4,000.00 on up

70-200L2.8 IS or 300L2.8 for under $4000. Over $4000 400L2.8 500LF/4 or 600LF/4
Depends how serious you want to get??