Mixing an SSD and Spinning drive for best performance

Hello-

I have a large Postgres database on Ubuntu 12 (+100GB). I have an old,
SATA-based, spinning drive and a new SSD drive. I mainly do data analysis
on Postgres (big, ugly select statements). I see a few options:

1. SSD for data and OS. Spinning for temp space.
2. SSD for data. Spinning for OS and temp space.
3. SSD for OS. Spinning for data and temp space.

I'm thinking that option #2 is the best. But, I wonder if moving the OS on
the SSD will give me a boost. I'm not too concerned about boot up time or
OS responsiveness as much as getting more speed out of my DB queries. I
have plenty of space on both drives for the data and OS with room to spare.

Which configuration would you recommend? Any other suggestions to get more
performance?