I think that the thing to consider most in your budget is the lens first. Once you have worked out what lens you would be best with, then work out how much there is left for the camera. As far as I can work out, once you have a lens which is good for you, it is more likely that the camera will wear out/be replaced first. Also, a good lens can seem to cost more than a good camera. Not wanting to put you off, but one of the lenses I would 'like' at the moment, is around £6k, hah, no chance.
For 'wildlife' back-garden birds, I have a lens which goes from a 70mm focal length, to 300mm. For a starters, you would probably need at least 200mm to get a decent shot.
For people shots, you will need somewhere from 17mm for groups to 28mm, for portraits, anywhere from 30mm-85mm
This is considering you are likely to get a 'budget'/'amateur' camera, which most of them have what is called a crop factor, greater than 1. This factor is not something that you need to get too concerned of now. The budget canons (the XXXXd, or XXXd range [and the semi-pro XXd]) are generally 1.6 crop, the nikons, I believe are varied between 1.3-1.5. The crop factor (at this time) can basically be thought of as a multiplier for the focal length of the lens). So when I shoot with the lens set at 300mm on my camera, which has a crop factor of 1.6, in reality it is as if I am using a 480mm lens.
If you go the Nikon route, you need to take a little bit of care I believe as to whether the auto-focus motor is in the camera or the lens. I believe that almost all current nikon lenses are compatible with current nikon cameras, but some will not auto-focus.
For the canon route, the 'budget'/'amateur' cameras support a lens type of EF-S and EF, whereas the 'pro' bodies (the full frame variety, again, don't worry about what this really means, but the crop-factor is 1), will only support EF lenses.
If you have decided on a lens/lenses, then perhaps a cheap
second hand camera is the best next bet. Lenses, if they are a 'good' lens, will keep their value over time. Cameras on the other hand, are always being superseded by the next best camera. Usually there is a rush to get the next number of megapixels, which means that the previous version becomes cheaper.
That isn't to say, that if you are taking photographs to print out A0, that a 25MP camera isn't going to help a lot. With a 6-10MP camera, a good/semi-good shot will print out OK at A4, which is possibly enough for you at the moment. The advantage of more MP later on, comes with things like small birds, if you have a lens which is good-enough, then you can effectively digitally zoom into the picture as well (crop to size).
I am not suggesting ebay, but the canon 400d (which is what I have), is currently going on there for between £250-£330 new. This was superseded by the 500d (this year?).
I think that the 350 or 400 are probably a good enough start. The 500 has more MP, [and has the gadget factor of doing video], but are these really required? I am sorry, I don't really know the Nikon equivalents, but I suspect that the d40 mentioned earlier is just as suitable.
