The differences between trhe 400 n 450, are the battery on the 450 is bigger but then this is needed if your using the live view(the teh screen) as this uses more energy. You should still however get a longer battery time with it.
The picture quality has moved up from 10.1 to 12 mega pixels which will make a small diiference.
The memory cards have changed from a compact flash to a large sd card. This is handy if its handle by less than nimble people. My rents just managed to break the pins in there 350D thru overexcited entering of the compact flash card.
Tehres a few other little changes here n ther, mainly to do with sensors, AF points and metering modes tho.
hope this helps, but as Farno siad go down to ur local camera shop n have a feel!
Paddy
(400D n very happy with it)