JDBC autocommit versus own commits performance

I'm using Postgresql 7.1beta4 with JDBC, and I was wondering if I should go to the trouble of modifying my connection pool to manage two pools, once with setAutoCommit(false) and the other with setAutoCommit(true).

As it is, I turne off auto commit since in my transaction processing, I need to control the commit/rollback.

But, there are also a lot of cases where I'm doing a simple query and I don't need transaction isolation, per se. Is the performance better if I use auto-commit than if I SELECT and do a commit() myself? And is there any benefit to doing a rollback on a select instead of a commit, since nothing changed?

Would there be a benefit with auto commit off if I had to do several different SELECTs, since I'd only have to do one commit at the end instead of the autocommits after each step?

Thanks,
David