Hi steiny03 and welcome to Pixalo. The 400D is the UK version of the XTI (US version) and they are identical cameras. I don't know where you are but if you live in the UK and buy an XTI then your warranty is only valid in the US and vice versa.
As to what camera to buy? I could suggest dozens but it would help to know something about what you intend to do with it and what is your price range. Do you want a DSLR or might a bridge camera be better. If you could post more info that would help. The major players, being Nikon and Canon, have a good range of DSLRs between them but other manufacturers are entering the market with some excellent stuff at competitive prices such as the Pentax K10D and the Fujifilm S5. Have a look at our
Equipment Reviews to see them all. HTH for the moment.
Edit - if you are going to buy DSLR you also need to consider what lens system you will be buying into since it is likely you will have your lenses longer than your camera.