Chances are you have either a Shimano Alivio, or Suntour XC-E stamped crank utilizing a square taper bottom bracket.
Check Sheldon Brown's/Harris Cyclery site for bottom bracket spindle length info once you've verified the model. Then you can check if a new crank will work with the old BB.
Personally, I'd buy a new matching BB for the new crank.
Check out Nashbar.com for some deals.
I picked up a Shimano FC-M340 triple crankset for $10 new. It is steel rings, but are bolted to an alumimum spider.
You need a thread-on crankarm puller.
Go to Park Tool site for pics and how-to info.