The title is pretty explanatory but not fully. "Best" for what? Linux is the most versatile piece of operating system on earth. The same kernel powers super computers and mobile devices (listen Linus' Aalto Talk http://moodle.org/mod/forum/discuss.php?d=210640 between 0:38:50 - 0:41:10). So Linux can power any version of Moodle with ease.
That said, even Linux can't do wonders. If the CPU is a 386/16 MHz, it will remain 386/16. You can't expect that to pull the current Moodle (2.3), which is expanding at an alarming rate. Go through the recent discussions here to get an idea of the kind of hardware it demands, for example https://moodle.org/mod/forum/discuss.php?d=212411.
Missing information 1: What is your hardware? Is it the same "very very" old computer you were talking about here https://moodle.org/mod/forum/discuss.php?d=213539 ?
Missing information 2: What is the future load? Read the compulsory performance documentation mentioned in the introduction to this forum to get an idea how to express that.
In any case, invest some time in the initial information provided in the introduction to this forum.