" would compiling be sped up if I made a ramdrive to extract/compile sources on?"
Probably not, because in creating the ram drive you would be removing memory from the cache. What speed you gained from the ram drive would be lost to a smaller cache size.
" I noticed on compiling that my cpu rarely gets over 75% usage, and my memory never quite fills up"
Linux always sets aside a little memory for emergency use. It would still do so if you created a ram drive.
In any case it would be interesting to try the experiment of running the same compile without a ram drive and with a ram drive and compare the two compile times.
Be prepared. Create a LifeBoat CD.