Help Me Deicide If I Should Buy This Bike

Great bike, but a wee bit on the heavy side.  I don't think the components (Dart fork, mechanical disc brakes, ) are going to fare well in an Xterra or adventure -style race.  Just my .02

By getting a lighter bike, will I be likely to spend quite a bit more?

Short answer...YES.  I guess it's not the weight so much as the fork and components (SRAM X-5 is good for casual riding, but I wouldn't race it-lot of plastic/resin).  But that Dart fork HAS to go.  Tiny little 30 mm fork stanchions (flexy), 100 mm travel is very, VERY optimistic, plus it's a coiled spring fork, no rebound, marginal adjustability.   Could you race it-hell ya.  Win on it-sure, if your engine is good.  I'd ook for something older with at least a RocShox Tora for something by Fox.  Manitou stuff is getting hard to find parts for.  Don't know about the other suspension choices. 

Do gears and everything matter as much on a mountain bike as compared to a road bike?

The ability to shift gears quickly and accurately is as important, if not more so, on a mountain bike as on a road bike.  MTB's also take a heck of alot more abuse which will knock lower-quality components out of tune much faster than higher quality.