Small MB?

I am working on a project to convert an old Mac +, into a Linux box,
instead of converting it into a fish tank. I've got a small VGA monitor that
will fit right in the case, but am looking for a small form factor mother
board and power supply. I'd like to have at least a Pentium class processor,
but not something that puts off a lot of heat and consumes a lot of power.
Board would have to be about 10'x10" or less.
Any Ideas?