I'd like to find out, what would be the best way to build a small form factor pc, for about 100$ or so.
What would be the best choice in hardware?
I honestly don't think it can be done for $100. The only thing I can think of is to get a set-top box like the Logitech Revue, but even then that's not a full PC.
You could try picking up a Zotac Z-box or something...but building your own would definitely require at least a few hundred...and that'd be for something with limited capabilities.
EDIT: you might be able to go cheaper from other sources, or you could possibly re-use old hardware to cut costs if you can find it.
EDIT: some links
EDIT: the CF to Sata adapter would have to just be strapped to something inside the case since there are no slots. Not a big deal. For a few dollars more you can upgrade to a HDD, but cheap was my goal. You could also save money with a Via based motherboard, but I have no experience with them and don't know if they work with various linux distros.