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Do you really want underscores rather than hyphens as separators? That defeats one of the SEO objectives. Google said they were going to treat both as equivalent but I don't know if that has happened yet.
In Joomla 1.5 you can go to Global configuration and there's a checkbox for "Search Engine Friendly URLs"
This is in 1.1 as well. It does not give you complete across the board search engine friendly url's, it only works on basic Joomla functions. You will need a third party solution. Check out Joomla extensions [extensions.joomla.org].
As anallawalla noted, sh404SEF is one of the better known ones for 1.5