I agree with Capwater. Motorcyclist, just because he's been lucky with his dumpster finds, and you can't afford a nice bike, it doesn't make what Capwater said insulting. You've received some good advice in this thread. A new bike would make alot more sense, even if it was the cheapest new bike in a quality bike shop, it would be alot better than what you have. The only attitude I'm detecting here is yours. You aren't doing us any favors by asking questions on the site, so if you don't appreciate honest answers, don't ask any questions.

To put things in perspective, I just thought long and hard about whether or not it was worth it for me to upgrade to all new components on a 4 year old bike that was originally $2000. I bought a new bike and use the old for a spare. New bikes, even the ones in the same price range as older bikes, benefit from new advances in technology.
Advances you would be in for with a new bike (assuming you get another mtn bike, even though I think a road or hybrid might be better for your desires):
frame material
disk brakes
threadless headset
wheel design
number of gears
shift lever design
etc.

Are you sure those cranks are plastic? I've never even heard of that. Is it possible they are aluminum that has oxidized and look like plastic? If that bike has plastic cranks, and you do upgrade it, chuck those cranks first and get the cheapest METAL cranks you can find. Plastic gets brittle with age, sounds like a recipe for disaster.