IMO, 400D just had some minor improvement over 350D (minor in
the sense that you can get a 350D and still be happy about
it).
Increased sensor pixel-count, anti-dust etc. The only major
improvement I feel is the increase in LCD size, from 1.8" to
2.5"
Other recommendation might be to get Nikon, D40, D40x or D80, I
shall not elaborate on Nikon since I'm not really familiar with
their system.
And if possible, dump the kit lens and get a better lens. You
mentioned taking pictures of cars, so a fast lens with narrow depth
of field might come in handy.
