Overhead cost of Serializable Snapshot Isolation

I'm looking into upgrading a fairly busy system to 9.1. They use
serializable mode for a few certain things, and suffer through some
serialization errors as a result. While looking over the new
serializable/SSI documentation, one thing that stood out is:

http://www.postgresql.org/docs/current/interactive/transaction-iso.html

"The monitoring of read/write dependencies has a cost, as does the restart of
transactions which are terminated with a serialization failure, but balanced
against the cost and blocking involved in use of explicit locks and SELECT
FOR UPDATE or SELECT FOR SHARE, Serializable transactions are the best
performance choice for some environments."

I agree it is better versus SELECT FOR, but what about repeatable read versus
the new serializable? How much overhead is there in the 'monitoring of
read/write dependencies'? This is my only concern at the moment. Are we
talking insignificant overhead? Minor? Is it measurable? Hard to say without
knowing the number of txns, number of locks, etc.?

--
Greg Sabino Mullane greg(at)endpoint(dot)com
End Point Corporation
PGP Key: 0x14964AC8