*nix on "classic" systems

[Brad Parker]

>> Does somebody know of a modern Unix system which truly focuses on
>> resources consumption, which is lean and fast?

There are several linux projects, most recently for 2.6 which focus 
on absolutely minimal ram usage.

   http://www.selenic.com/linux-tiny/

"damn small linux" is also pretty small disk wise.

  http://www.damsmalllinux.org

>Especially, I'd like to hear about an open-source C compiler that (a)
>supports at least sparc, i386, mips, and arm - those being the CPUs I
>care about at the moment - and (b) is not a resource pig.

I assume you've seen tcc

  http://fabrice.bellard.free.fr/tcc/

-brad