Dilemma

After over 3 years of using a Powershot, I thought of graduating to a SLR. I planned for a decent SLR but had no plan on lenses.. well I never had taken that into consideration.

Then Ranthambore trip happened. I went crazy over what to buy..from a TC for my Powershot to all possible cameras that I can buy at that point of time, finally ended up buying a Canon EOS 1000D and with some good advice, a Sigma 70 300 APO DG MACRO F4 – 5.6.

Ranthambore trip was a great learning experience, my first wildlife shot was of T39 tigress. I got so confused on either to take pic or watch the tiger go just feets away from me. So did not pay attention to details. Rest of the days in Ranthambore were good, learned a lot from wildlife photographers(Sagar Gosavi and Yogesh Rane).

Next trip was after a week from Ranthambore was Jim Corbett National Park and this time I tested my setup as a camera person . Well I found myself starting from scratch here in Corbett.

My few observation in regards the last two trips were :
1) Canon EOS 1000D is an awesome camera but not at all suitable for Wildlife photography.(1.5 fps in RAW is way too slow).
2) OS/IS/VR is somehow a requirement for early morning and late evening shots(Most movements happens in this time though).
3) ISO the more the better. Noise the less the better ( 1000D scores a good point her though).

In the last two trips I narrowed down on my short comings trying to fix them but still I need a basic setup to do wildlife photography(I might be carried away with the crowd though). Here is what I shortlisted and currently in a dilemma.

1. Canon vs Nikon : Being a faithful canon user for last three years I wanted to stick to canon but found in some cases Nikon provides exactly what i want.
2. Lenses (Sigma,Canon,Nikon ) : Sigma cheap and does great job, while Canon/Nikon are better and costs a lot

Finally these are my shortlisted cameras and lens :

Canon EOS 550D : Great camera with great feature but misses on fps.
Used Canon EOS 40D : Again Great Cam but 10MP(low resolution on high crops, ISO to 1600 only).
Nikon D5100 : Pretty much does everything but I have one or two inexpensive canon mount lenses which goes wasted if I chooses this.

I pretty much narrowed down on Sigma 150 500 F5 – 6.3 APO lens which means I need high ISO a lot of times.

Taking all into consideration I am confused like never before :-(

So please help!!!!