Thanks, still weighing up loads of options at the moment, so I'm going to try them all as soon i can, so i can finally decide which one. Good to hear that the 450D and the 55-250mm lens work well together, thanks for the advice about the other lens aswell.
Matt, Canon and Nikon offer the widest selection of equipment, plus many more third parties also provide lenses etc. in their mounts too, but don't less this put you off the other brands. You'll find you can cover the whole focal length range from any of the major brands and still build up enough equipment to cover every possible photo opportunity. Each brand also offers a range of different quality lenses, from your standard level to the finest pro quality, so as you progress you can upgrade your lenses to better ones as your budget allows, though with some brands you will also need to upgrade your camera body too, to be able to use their very best lenses because they've been designed to work with a larger sensor format.
Sensor size is probably one noteable difference between the brands. The likes of Canon, Nikon, Sony and Pentax all use the APS-C 1.6x crop sensors, whilst Olympus and Panasonic both use the 4/3rds 2x crop sensors. Put in english this means, if you measure the size of the sensor against the size of a negative from an old 35mm film camera, the sensor is smaller and the difference in size is measured as a ratio of either 1:1.6 for APS-C or 1:2 for the 4/3rds. So a 50mm focal length on an Olympus DSLR camera would be the same as a 100mm focal length lens on a 35mm film camera or with an APS-C a 50mm lens would equate to an 80mm focal length on a 35mm camera. Now pro DSLR's like the Canon 1D Mark IV are classed as full frame cameras that have a sensor that is equal in size to a 35mm negative. Now you're interested in taking pictures of wildlife, so a 250mm lens on an Olympus or Panasonic would give you the narrower field of view of a 500mm focal length as opposed to giving you 400mm focal length with an APS-C sensor so it would get you closer to your subject.
Olympus and Panasonic have developed this smaller sensor because they feel that in the future people will want to buy more compact and light weight cameras, but without compromising on the quality. To do this, their lenses have to be telecentric, which means as the light passes through each element in the lens it straightens it so by the time it hits the sensor it's at 90 degrees to it, giving you a sharper image in theory. Light however, often bounces around inside the lens and can lead to distortions like chromatic aberations and ghostings appearing when shooting in certain light conditions, which is why they've gone for these telecentric lenses. The downside to a smaller sensor though, is when you're working in lowlight conditions you cannot get as much light on to the sensor because of it's smaller surface area and noise will more readily appear in the shadowy areas of your images. The APS-C sensors tend to cope much better with noise, particularly when using ISO above 800, but they also tend to be a bit bulkier and heavier. It's a trade off I suppose.
Now you mentioned the Olympus E450, which is a great little camera, except for it's sensor and lack of any image stabilizer. This means that when taking any photograph you'll have to remember that whatever focal length you're using you'll need to times it by 1.5 to workout what the minimum shutter speed you can use to stop any dreaded camera shake blurring your images. Or you could buy the E520 or if your budget would allow the E620 that both have image stabilzers built into the camera body, as they don't have them on the lenses. I always tend to use a tripod anyway, particularly for landscapes to help me with this.
Anyway, best of luck and take a look here for the current best deals on the market.