From Moodle 2.5, there's a new tooltip module - moodle-core-tooltip. You can see it in use for help popups on http://qa.moodle.net. The popuphelp implementation is set to use the click event handler for the target link, but you could theoretically use a hover too.
The other thing to consider is accessibility though - how will a screen reader present this to a blind user?
I did the same in Moodle 2.2 long time back.. i used this yui gallery module
you can always use YUI IO in order to get the content for the tooltip from a remote source,couldnt you ?