Best use of second controller with faster disks?

Configuration
OS: FreeBSD 6.1 Stable
Postgresql: 8.1.4

RAID card 1 with 8 drives. 7200 RPM SATA RAID10
RAID card 2 with 4 drives. 10K RPM SATA RAID10

Besides having pg_xlog in the 10K RPM drives what else can I do to best use
those drives other than putting some data in them?

Iostat shows the drives getting used very little, even during constant
updates and vacuum.

Some of the postgresl.conf settings that may be relevant.
wal_buffers = 64
checkpoint_segments = 64

If nothing else I will start to put index files in the 10K RPM RAID.

As for the version of postgreql.. we are likely getting a second
machine, break off some of the data, change programs to read data from both
and at some point when there is little data in the 8.1.4, upgrade the 8.1.4
machine. The new machine will have 8.2.4

We have a lot of historical data that never changes which is the main
driving factor behind looking to split the database into current and
historical.