I would hesitate before beginning a wholesale component upgrade. You can get a fairly nice new MTB these days with disc brakes for the same money a new crank, front and rear derailleurs and a wheel set will cost you. As long as the wheels are still true and spinning freely on their bearings I'd stick with a tune up.
You don't need a new crank, but Does it creak or clunk when you stand up and pedal? Take the chain off and spin the pedals around. They should rotate at least a half dozen times, given a good push. If they drag, wobble or clunk you should replace the sealed bottom bracket. It's not hard and lots of folks here can advise you.
New cables and chain will make a world of difference. That particular bike probably came with OK brakes. You might consider replaceing them with a new set of OK brakes. Really, you can get a new set of V-brakes for like $10 or $15 a wheel - you don't have to spend more than that.
Unless the bike's got a trillion miles, or has seen some serious muddy, noreaster rode gritty winter duty, I'd guess the chain rings and sprockets are alright. You're a mechanic? Inspect them for wear. If you're running the original chain it might be stretched out enough to be causing you gear jumps. And if you've never adjusted the derailleurs that might be the reason also. However, my experience with derailleurs is that they last a long time, especially front ones. I think you'll get a way with cleaning and adjusting them. And again, there's lots of help here doing that.
Start with that and see if your bike's performance improves. It woun't cost you much to see if there's still life in the old girl. If you start looking at more than $150-$200, look around for a good deal on a new bike.
DanO