Well...As you say, Huffys are really low-end bikes meant to look good to young kids and hold together for a season or two. The frames tend to be very heavy mild steel, and the parts the cheapest that can be obtained.
Putting better parts on one of this can be done, of course, but you would rapidly blow more money than the bike could ever be worth.

My suggestion: Save your money and go looking for a decent used mountain bike. There are pretty decent machines available new for around 300 dollars, so you should be able to pick up a used one for 100 bucks or less.
Keep your eye on yard sales, thrift stores, and places like that.