Not sure.  The 1.6x senso bodies at Canon have weird
lens requirements.  In principle you should be able to,
but better check with Canon themselves?

No.

Yes... All Canon DSLR will accept all EF lenses.  The 400D will also
accept EF-S lenses.... these are cheaper lenses designed for the 1.6x
bodies only.

Cheers

Russell
My Photography Help Blog: http://blog.pixelpix.com.au

Might be worth keeping the film body for times when you aren't allowed to
use the digital body - such as take off and landing on an airliner (which
happen to be my favourite parts of the flight as far as photography is
concerned).

I have Canon 35mm, APS and digital SLR bodies. The EF lenses work on all
those bodies.
Caveat would be older Sigma lenses (second-hand) will probably work on the
older film body but not necessarily on the new digital body. It's a Sigma
thing and the cure is to have updated chip into the lens.

In the reviews of Canon versus Nikon that I've read, the result is usually
that the makes are very close in performance and that it is not worth
swapping from one make to the other if you already have bodies and lenses
for one of those makes.

As you have only a couple of Canon lenses then you aren't very deep into the
Canon system so you have not got a big overhead if you do move over to
Nikon. See if your retailer will let you have a "play" with each body for 20
minutes at least. A friend and I did this with the Canon 30D and 350D and we
actually had several sessions (typically 15 ~ 25 minutes each time) before
we made a choice with which we have been very happy.

Regards,  Ian.