What type of photography do you do? And what attracts you to an SLR?
The main flexibility of an SLR, IMHO, is the fact that you can interchange the lenses to cover any scenario from taking portraits of ladybirds to motorsport, airshows, etc. As your collection of equipment grows your lenses will become the most important (and, probably, valuable) asset.
To my way of thinking, therefore, the choice of camera body relies on easy availability of lenses (whether they be new or 2nd hand), and that makes Nikon or Canon the no-brainers on your list. Even if you grow out of the camera body, buying into a well-supported lens system is important. Once you've collected a few lenses you want to know the system will give an upgrade path for the body to use with your lens collection.
The Pentax K-m might also be worthy of consideration at that price level. It's about to be discontinued without a successor, unfortunately, but there are likely to be a few bargains around because of that. Auto focus is not the best on that body, but it does have image stabilisation in the body so you won't have to rely on lenses with it. If you can stretch a bit more I had a play with the k-x recently and it's very capable, although quite a bit more money. Picture quality is up with most £1k+ bodies, IMHO. AF is much better and it does video. I ended up splashing out on a K7, though

(mainly for ergonomic and build quality reasons. I reckon the k-x is up there in terms of picture quality).
Any of the above choices will give you access to plenty of lenses either new or 2nd hand, and reasonably priced, especially if you are prepared to live with older manual focus lenses.
Don't disregard second hand bodies either. Most SLRs are robust if looked after and very reasonable 2nd hand as most photographers suffer from acute upgradeitis

. You can get a lot for your money 2nd hand and if you buy from a dealer you'll probably get a warranty too.
On the other hand, if you see yourself being satisfied with one or two zoom lenses, and not needing a collection of glass, you need to focus more on what camera you are getting for the money, and can cast your net a litle wider and consider the likes of Olympus and Sony and more. If size is more important than ultimate picture quality and flexibility some micro 4/3rds cameras are probably worthy of consideration.
Just my 2 penneth. You'll hear plenty of different opinions.

Kevin