I have done some upgrades the the mtn bike over the past couple of weeks.
New Brakes: Shimano XT - much better than the stock avids.
Bars: Easton Haven 740mm - who knew how much better wider bars were. All my old bikes I cut down the bars to shoulder width.
Stem: RaceFace Turbine 60mm
Grips: Easton lock-on
Bars and stem change really changed the handling for the better. More stable both uphill and downhill and with the short stem it is easier to get the front end up when going off a drop or over and obstacle.