RE: Batterys
Not a stupid question at all. LiPolys are expensive, and picking the wrong pack for a particular application can be costly.
Whether the LiPoly will be a good replacement for the NiCd depends on the situation.
While the LiPoly has a little more voltage, and almost as much capacity as the NiCd, it's also physically much smaller, and can't handle near as many Amps as the NiCd. Try to put that pack in a 1/10 scale buggy, and you'll turn the buggy into a smoking heap in no time. The pack will overheat, and probably burn inside the car, because the load is more than it can handle.
The LiPoly might work well as a replacement in a smaller airplane, but again, it depends on the plane.
What in particular are you trying to power with LiPoly.