Joomla just released the security maintenance version 1.5.20 today. If you just upgraded your Joomla to 1.5.19, you are very urged to upgrade to 1.5.20 because there is an urgent security fix in 1.5.19. If you haven't upgraded to 1.5.19, then you can either upgrade directly to 1.5.20 or wait for a few days until it is sure that there is not too many problems arise with 1.5.20.

As for our template, there should be no effect with all this upgrade.

http://www.joomla.org/announcements/release-news/5284-joomla-1520-released.html