I have 2 850s and have replaced 4 thermostats so far (you get about 80,000 to 100,000 miles before they fail)

If you don't mind a little spillage - maybe a 1 cup, less if you are quick - no need to drain coolant as on http://www.matthewsvolvosite.com/tstat.php Although if you haven't done a fluid change lately not a bad idea. But if you are looking to do change the t-stat quick only takes a few moments to do it (but wait for engine to cool first!)

And I agree that a torx socket bit much better then screwdriver type and less likely to strip the torx bolt head.

Good luck!