+6 to 8 on the above.

Component level is just one of the shortcomings of many (not all) department store bikes. As mentioned, you have a bike that is worth upgrading and will last you many years. Don't get all excited and start upgrading willy-nilly though. If a component is working, leave it alone except for routine maintenance. As things get worn or otherwise fail, upgrade by a component group or two such as replacing a Tourney derailleur with Alivio or Deore. Wheels are a good investment but can get expensive. If you find your wheels coming out of true or breaking spokes frequently, then it is time to upgrade. You should be able to get into a set of 36-spoke, double walled rim, on decent hubs for around $200. Look for wheels designed for touring or cyclocross, they will be tougher but a bit heavier than "road" or "racing" wheels. If you have a local bike co-op, or contacts with other cyclists, you can sometimes find good midgrade wheelsets fairly inexpensively when someone upgrades to higher end components. I get a fair number of Alivio, Deore, 105 and similar level components this way.