Try reading up on some sites/ forums like
www.dpreview.com,
www.fredmiranda.com etc - you'll find a lot of details there.
If you're looking for a solidly built (= quite heavy) DSLR with a decent size hand grip and a few more features, the 30D is great. You may find it balances better if you use some of the heavier lenses too.
From what seems to be emerging on the 400D (only 1 review so far), it looks like it will continue the Canon tradition of being an amazing quality and value DSLR. If the smaller grip, lighter weight and slightly reduced features are ok - I would get this and spend your savings on lenses.
Phil
(30D owner)