Sorry, this seems ill-advised. Joomla 1.5.23 is not current, and the updates since then were probably security patches, Joomla 1.5 is scheduled for End of Life in a few weeks. That means no more development, not even any more security patches. It doesn't make sense to struggle with outdated versions of both Joomla and Civi, then an upgrade later.
You may as well struggle with Joomla 2.5 and Civi 4.1, at least then you'll be on the latest platform. That way, you and the Community can focus on moving forward, not looking back.
You shouldn't be stuck on 1.5.23 because any decent host has updaters like Fantastico, Virtualmin, Softaculous, etc. If you host with Cloudaccess.net, you'll never need to worry about updates.