Don't bother with XD
For me XD falls in the same camp as Sony memory stick. Unnecessary. I don't see why we needed yet another format that offers no real advantage and lower memory sizes for a higher price.
I would have to say go with either SD or CF. Those are the two formats that have the widest acceptance. I use my 2GB CF card between my camera, laptop, desktop, printer and PDA. None of the other formats are supported to this degree.