I use the Lowepro Edit 140. It's a great little bag and I can fit all the accessories in it along with the 400D and a spare lens.
It's even got a little space left over I could fit a couple of filters in.

Mate of mine has a 400d in a Lowe Rezo TLZ20, which just about fits the cam, lens and cards nicely. I don't think you'll find much smaller that will fit the bill.