Having a quick look at prices, I think the XDs and XXDs might be out of my price range. I would prefer to buy new if possible. Has canon followed their numbering pattern at all ie is a XXXD better than a XXXXD? Is a 250D better than a 400D?
It depends what you think of as 'better'. The 400D was a decent enough DSLR in its day (I had one too) but in terms of low light performance, dynamic range and megapixels it really is old hat by today's standards. Not that you can't still take a good looking photo with a 400D... it's just that it's easier to consistently take a good looking photo in less than ideal conditions with a modern DSLR.
Generally speaking, the lower the number the better the spec, with Canon... but that is in comparison with other cameras in their current range, so do check the spec and features of your 400D against whatever you are thinking about, to make sure you're not losing any features you currently use and like when looking at a replacement.
As for price, don't dismiss buying a used DSLR in mint-ish condition with a low shutter count (lightly used) from a long-established and reputable dealer, as you'll probably move up from a XXXD to a XXD by doing so, giving yourself better value for money. I think Wex and Harrison's Cameras (in Yorkshire) both give a 12 month warranty on most of their used DSLRs as standard, which is all you'd get from most approved Canon retailers if you bought brand new (apart from John Lewis who usually give a 2 year warranty). Over the years I've bought new cameras
and used ones. I just look for the best value for money - if I can save a good chunk, or move up a model, and get a good, long, warranty, then I'll go for
second hand.
Some people buy a new camera and find it doesn't suit them, others buy a new camera and subsequently discover that they can't really afford what they've spent, then they end up selling it after barely using it. Over the years I've saved a good chunk of cash by buying lightly-used (virtually mint) camera kit from reputable dealers, so do give it some thought, particularly if it enables you to move up a model range and you can genuinely make use of the quality and/or features this brings.