Little tiny server (Mac Mini Server clone)

Hi, I want to make something like a Mac Mini Server clone. Just for a basic web+mail+file server. Perhaps to stream some media, but not HD and it won't be connected to a TV or anything like that. I will run Debian Lenny for the operating system.

It doesn't need to be particularly powerful, just good enough to run a couple of low traffic static websites, a mail server for a few people, and a big-ish hard drive. The MAIN requirement is that it has to be small. Preferably as small as a Mac Mini, and thats what I'm having trouble with.
All the mini-itx cases I see are actually much bigger than a Mac Mini, any ideas on cases? It doesn't even need a CD drive.

Also, will Atom be powerful enough? I want it to be low cost and use minimal electricity, but it needs to function obviously.

Any ideas?