This is something I'm currently dealing with.

How Stuff Works .Com is VERY useful for info like this.

Piston rings keep combustion gasses from going past the piston, and the oil from entering the cylinder.

"Piston rings provide a sliding seal between the outer edge of the piston and the inner edge of the cylinder. The rings serve two purposes:

They prevent the fuel/air mixture and exhaust in the combustion chamber from leaking into the sump during compression and combustion.
They keep oil in the sump from leaking into the combustion area, where it would be burned and lost.
Most cars that "burn oil" and have to have a quart added every 1,000 miles are burning it because the engine is old and the rings no longer seal things properly. " -HSW.com

Depending on how bad yours are, you may be fine driving it for a while. I also have a bad head gasket, which is causing my car to overheat, so I HAVE to change my gasket, and while I'm at it, I'll do the piston rings. Neither is a small job, but you have to remove the head to get to the rings, so it makes sense to do it now rather than later.

While I knew it would be $200+ in parts to do this, I wasn't quite counting on all the tools I would need to buy. But...I'll have them for next time (;

I doubt that using lighter weight oil would cause them to wear, but anything that is easier to get past the worn rings, will result in more loss of oil.

If you intend to keep the car, or can't afford a newer one, it's something that should be done if you have the time. If you don't forsee keeping the car long, and can keep an eye on the oil, it's probably not worth it.