I use a Canon 400d, which is absolutely brilliant as an amateur SLR, and is available for fairly cheap now. The 450d is also a stunning camera and does have some noticeable improvements over the 400d (live view, larger viewfinder and spot metering to name a few) so well worth the extra pennies.
Really though, You can't go wrong with any of the big names nowadays. Canon and Nikon have the benefits of a huge lens range and solid reputation for digital SLR's, which brands like Olympus, Sony and Pentax don't have the same clout behind them, but are all brilliant cameras - if you are looking at SLR's though, it is really worth researching them all as a system, not just as a camera. Have a look at the different lenses and accessories available and see which suit what you like and would eventually want to buy. Once you have bought in to a system and bought anything like new lenses it will cost a lot more to swap to another brand and replace everything, so consider them as a whole.
On the point about changing lens mount, Canon have had two SLR mounts, the older manual FD mount, and the newer electronic EOS mount. When they changed they did alienate quite a lot of people, but have more than made up for it with the lens lineup available since, and you know that anything with an EF mount will fit any Canon EOS camera (so thats basically anything from the last twenty years, including the 19 year old lens I recently bought which works perfectly.)
Nikon on the other hand have stuck with the same mount, but in recent years have changed some details in how it works and limited the lens choice for some newer cameras like the d40. I don't know exactly what is what though, the Nikon I own is from the 1970's
so hopefully someone more knowledgeable could chime in.
The best bet is to get yourself down to a few shops (Jessops, Lizars, John Lewis...) and handle some of the different cameras, see how they work and fit your hands. At the same time have a bit of a look into the lens lineups and accessories available and see what fits what you would like to use.