Moving some of Postgres off a SSD

I have Postgres running on a SSD. The data is now almost 50GB, which
is filling up the drive.

How can I move some of the data to my HDD?

My priorities are, in this order:
1. Reliable - I don't want anything that will corrupt the data
2. Easy - I have a few dozen databases, I don't want to do too many commands
3. Speed - If possible, I'd like to keep the key stuff (perhaps only
the WAL) on the SSD
4. Space - Ideally, I'd like to move all of the low priority databases
- about 30 GB

I can take the system offline freely for as long as I need.
(Postgres 8.3 on Ubuntu x64)

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