First, there is no harm in really cranking down on the crank arm bolt to see if it will tighten up. If not, then you need to decide if you want to change it yourself or pay to have it done. You'll need a crank arm puller ($10 or so) and, assuming you'll do the bottom bracket as well, you'll need the apporpriate tool for that (again, around $10). You'll also have to do some minor adjustments to the front derailure once you get it all together but it shouldn't be a big deal.

There are a few important numbers for cranksets. Number of chain rings (assume yours has three so it is a triple), number of teeth on each ring (Bikepedia shows yours as 42/32/22 but that's easy to count or look for it stamped on the rings) and crank arm length (distance from center of spindle to center of pedal shaft - typically 170mm or 175mm on MTBs like yours).

The crankset determines the length of bottom bracket spindle needed so you'll probably need to replace that as well. If you went with a Shimano Alivio crankset it would be in line with the stock components on your bike and should be well under $50. I'd look for a crankset with removable rings (that usually eliminates the really cheap ones) and match up the other numbers to what you have.