The 400D unlike the 30D has a 10Mp sensor , and the Sensor cleaning It is smaller too
It shares with the 30Ds its 9 point AF system and 2.5" LCD ( the 400Ds is brighter but it is also used as a status LCD .
They both have the Digic II image procesor
On paper the 30D doesnt seem like an upgrade
The 30D is much faster in use ( bigger buffer) and although the electronics are similar seems to have better AF
Its sensor is slightly larger than the 400Ds , as such the fewer photosites seem to get a sharper " per pixel" image
The 5fps you may not need nor the spot exposure monitoring but it really is fast
As an aside the 40D is said to have a similar 10Mp to 400D but if the comparison of the 350Ds 8Mp and 30Ds 8.2Mp is anything to go by, it will be better implemented
The 30Ds low light ability and lower noise characteristics are much better than the 400Ds
Paper specs and matter of factly opinions only tell half the story. I certainly rate it as a much better camera if only because of the ease with which you can access the settings ( it has a thunbwheel, big wheel and joystick!)
Its battery also lasts longer!
HTH