The cafe thing does hold a certain interest in that it would be possible
to give members their own logon account usable from any of the cafe
machines for reading (IMAP) email as well as having their bookmarks
stored centrally.
As for Linux gaming, there's always the Quake series, Unreal Tournement
(hopefully with better luck than I had setting that up), the standard
free Linux/Unix gaming such as bzFlag, FreeCiv, FreeCraft, Armagetron,
Xpilot, and definitely Crossfire.
You'll be hard pressed on hardware.  The guys who mention pricewatch
pricing are close, but they forget two very important issues... one is
shipping costs, the second is the fact that it's easier to return a
product locally when it's defective than to have to get an RMA# and end
up losing out on shipping each way.
Selling custom system guaranteed to be Linux compatible would be a nice
thing to have though.  Obviously, you'd have to be picky about which
distro you claim compatibility with.  You can go the proven Red Hat
compatible route, or claim compatibility with a distro that tends to be
a little closer to bleeding edge such as Mandrake.  (These are just
examples, people.  No need to light the flamethrowers over them.)