Huh, I didn't notice the 1-piece crank. Thats amazing on a bike costing that much. If you ever want to put a bigger tooth crank on there just look in some junk piles and you will find loads of cranks to stick on there for free!!! LOL Its a cool bike though from what I can see though. Like others have said upgrade stuff you can put on another bike. The thing is though generally speaking you can get a complete bike for less than upgrades. Basically ride this one to beat on and have fun with, but if you want to go fast save up some money and buy a 2nd bike thats a road bike. Thats my opinion
Here are a couple of questions.

What kind of cash are you willing to spend?
What kind of riding do you really want to do?
What is the market for used bikes in your area?

It sounds like you are having a ton of fun on your bike. Keep it up. =)