the minimum linux necesary for a computer to work
 

i want to put Linux in a Ram Disk from which to boot on a given configuration.it is very difficult to choose which files to select
as i wish the memory quantity used to be as small as possible.

There are a number of tiny distributions listed here, if you don't want to do it yourself.

http://en.wikipedia.org/wiki/Mini_Linux

Might look at LFS and put only what you need on it.

There are plenty of floppy based distro's. Might be able to run them with grub4dos if syslinux can't boot them.

You can use Slitaz for that, it contains already tools for building your own flavor of it. It is really an easy thing to do with them.

Sabayon is worth looking at. It has two special CD versions where you just get the kernel, the Gnu tools, X (in one of them), a text editor, and a package installer.

Tiny Core starts even smaller, with Busybox instead of the Gnu tools.

The only things needed to successfully boot and mount a root filesystem are:
/bin/sh, /dev/console and 'init', located either in /sbin/init, /bin/init or /etc/init. sh and/or init can even be a link to something else -most likely a linuxrc file. So, you can create a statically-compiled binary called linuxrc, create links from /bin/sh and /sbin/init to it. That gives you a (possibly) complete running system with any purpose -or none as you choose. One file, one device file and two links. Put that into an initramfs and compile it into the kernel and you have a 'complete' system from a single file.