Dude - I don't remember recommending a 400mm lens. Not sure that I've ever recommended that length to anyone because I know how expensive they can be. Oh, well...
Understand that you can also mount non-Canon lenses to your body. Sigma, Tokina, and Tamron are among the most popular generics. Do your research, though. You'll get more lens for your money, but the optics aren't always as good as what you find in a Canon lens. Each of the manufacturers that I listed make some very good lenses, and they also make some very bad ones, too. For example, Sigma makes an awesome 10-20mm wide angle zoom, but they also make some lenses that aren't worth the cost of the materials used to make it.
Be careful with teleconverters (TC), also. Like Gambino said you'll lose an f-stop or two. If your 300mm lens is consumer grade then it at it's widest your probably only able to achieve f4-f5.6. Take those widest f-stops away with a TC and you're left with a lens that is really only usable in very bright lighting. Also, a TC could soften your images, too. Remember - you're adding a few more pieces of glass between your subject and your sensor. Nikon makes 3 TC's (1.4x, 1.7x, and 2.0x). From what I've heard from many of the pros that I know, the 1.7x and 2.0x are nowhere near as sharp as the 1.4x. Since extending my zoom by .4x isn't really worth what I would spend I'll hold off on buying a 1.4x TC. However, I've heard that Kenko's pro line is worth looking into. I also noticed that you said that you might look at a Nikon next year. Before I invested another penny into a lens I would give that some long, hard thought. Anything that you buy that is compatible with a Canon mount will not work on a Nikon and vice versa.
As for your filter, why a polarizer? Are you looking for a filter simply to protect the end of your lens, or are you going for a specific effect?
With regards to the Canon pro bodies, personally I wouldn't drool over the 1Ds mk2. The Mark III is where it's at. The results that I've seen from this camera shooting at ISO6400 are simply unreal. Plus, it cost somewhere in the mid $4k range. Actually, the standard Mark IIn is still an awesome body. With the advent of the Mark III you're going to start seeing some steals on good, used Mark IIn's.
One final word of advice: Don't buy into the megapixel myth that says that more is better. You will see no difference between an 8.1mp camera and one that has a 10.2mp sensor. Follow this link to see why I say this:
http://www.kenrockwell.com/tech/mpmyth.htm
Hope that helps.
