Do not upgrade to Logic 9.1.2 if you're running a quad-core Nehelem Mac Pro or iMac. Apparently it cuts the performance in half. Insane!
http://www.gearspace.com/board/music-computers/540098-logic-9-1-2-a.html
http://www.gearspace.com/board/music-computers/540098-logic-9-1-2-a.html