You didn't mention your specific tires, but tires do vary widely in weight, and they're actually one of the heaviest single components on the whole bike. And they rotate on a large scale, so they should get priority when you're considering weight-reduction plans.
In the tire market, tires with Kevlar beads save a bit of weight over their steel-bead counterparts, and some manufacturers also use lighter casings that further differentiate their Kevlar-bead version. There are also lighter-than-average tubes available that may help reduce weight just as much as the light tires, so check that option out too. The WTB NanoRaptor Race looks interesting, at a claimed 510 grams: link to WTB's site
A typical steel-bead name-brand 2.1" tire is going to be around 600-650 grams, so that has some potential.
You might consider rebuilding your wheels too. RhynoLites are a bit overkill for XC unless you're not a finesse rider. As a quick-&-dirty estimate, if you presently have 32-spoke RhynoLites with 14-gauge spokes and brass nipples, and had the wheels rebuilt with Mavic X221's, alloy nipples, 14-15ga. rear and 15-16ga. front spokes, I'm estimating about 140 grams lost on spokes and nipples, plus 110 grams per rim, for a total of 360 grams lost. Combined with a light pair of XC racing tires, I think you'd notice a difference when riding. I used to have two sets of wheels, and the race-weight ones were noticably easier to climb with.
The STX-RC crankset is not very light, and uses flexy, suck-prone steel chainrings too, so you might also consider upgrading to a DeoreLX with the big-bore splined bottom bracket, to chop off some weight while improving functionality. My estimate is that the STX-RC crankset and bottom bracket are about 1100-1200 grams (educated guess), and a DeoreLX combo would be about 900 grams.
The steel seatpost is an easy 100-150gram weight reduction if you can get a plain old Kalloy Uno, a light seatpost that is under CA$30. You might consider removing the handlebar to see if it's some 300-gram monster too. The fork is not exactly light but that's one area where I wouldn't go gram-counting if it works well for you.
Hope that helps