Linux Directories to Map to SSD
If you're not installing software constantly then I'd map:
/bin
/sbin
/usr
/lib
/opt (if you have it)
/etc
to the SSD.
These directories are going to be read many more times than written to (as writes only occur to these locations when software is installed).
In addition to the directories listed by superdupersheep:
/bin
/sbin
/usr
/lib
/opt (if you have it)
/etc
I'd also add /boot, and also make the SSD the boot drive since this is where the linux kernel will live.