I am considering the upgrade from the 400d, to the 50d (or replacement)
The 450d is not going to accept the same memory cards.
However, both the 40d and 50d will take CF cards.
For me, this is a significant cost. I have 5 CF cards, and tend to use the camera a lot on longer holidays. This means having many cards is useful. There will probably be a £200 cost in replacing cards with SD.
Both of these are meant to be decent upgrades. 40D I seem to remember has live-view, same MP.
50D has live-view+more MP.
Main reason I am considering is supposed (well I have seen some great examples too) better handling of ISO. Don't know about you, but nowadays I cringe at going to 400 ISO on the 400d.