Besides my rear derailleur being Shimano XT, everything else, front derailleur, shifters, crank, and hubs are the basic Deore. I plan on upgrading these components one at time to the XT line, in what order should I upgrade these components? What makes the most significant difference? Btw, I have a 2007 FSRxc Comp. Thanks in advance!

ps. I'm thinking other components such as handle bars, stem, headset, etc. should come second to transmission components mentioned above?