I want to buy a digital SLR camera?

and am opting towards the Canon EOS 400D - does anyone know what lenses are compatible with this camera? Does it have to be a Canon lens or do lenses have universal fittings? The reason why i ask is that I have found loads of 300mm lenses on e-bay that are a fraction of the new price but none are made by Canon - thanks!

First let's get something clear. It's not the manufacturer that matters but the mount. Even if it's made by Sigma or Tamron, if a lens is specified to have a Canon mount then it will fit on your camera.

Other than that, even if you find a lens that sounds cool to you but doesn't have a Canon mount, chances are there are adaptors there that will help the lens fit. However, this practice isn't usually worth the effort since this way you will probably lose some of the lens' features (usually autofocus is lost, as the camera can't communicate with the lens on a different mount).

My advice is: don't jump to the biggest lens you can afford unless you know you'll stick to wildlife. Fast and long lenses are generally expensive, anything under $500 is probably usable only in bright light outdoors.

As for the camera choice, 400D is great for a starters, I would advise you to check out Pentax's K10D. For a little more you'll have a sturdy camera resistant to water and dust, more solid body. The loss will be in image quality at ISO over 600 and that is because the image in the K10D is made to be processed and used in editing, therefore there is less processing in the camera (400D's image processor makes a jpg image hardly usable in editing).