CPU Upgrade in a Mac Pro 1,1

Hi, i recently bought a 2006 Mac Pro (1,1) cheap for a "gaming" PC so to speak. Cheap, fast, well made, etc. my original plan was to chunk the old MB and install a new gaming one with an i5 or something. but i don't want to go that route as that requires too much DIY and homemade work and is costly. i can't find many places anymore talking about upgrading these, though i have seen many people swap in the X5365 3.0GHz units to make an 8-core, better than the i5 i was going to get. However, i really wanted to get a Harpertown 5400 series Xeon, a little cheaper, and faster. i know its the same LGA 771, etc., i don't need to worry about firmware and Mac OS compatibility issues as im running Windows (games as said above). Help?