I've got a 601GB Linux partition which can be written to at up to 84.7MB/s (ref). My 62GB can be written to at 366MB/s (ref).
Down to my last ~360MB. What should I move over and mount from my other drive?
The most common directories to move from an SSD and mount to a HDD are:
/home
/var
/tmp
/usr
swap
You should install your OS on the SSD. The /home partition will be on the slower hdd.
You could use the ssd as a cache drive in the raid utility with conjunction with your hdd. In that way your mobo (if supported) can automatically detect and cache in the ssd your most used data without you setting mount points and it works with any os and any partition you might use.
/tmp
&/var
too?