right. this is a good point to have brought up sv. when you buy rug pads, generally the thicker you purchase, the more it will insulate and both reduce noisel. Be sure your doors will still have clearance tho because that added cushioning also brings height with it....
There are some pads out there which can be quite an investment. Customers used to be shocked as in some cases, the padding options which were available were almost as much of an investment as the rugs they purchased. In my opinion, for most average and better Oriental Rugs, a pad is completely worth it - one of the more expensive and better ones are called durahold, it's purple/blue with a hexagon non-skid print. perhaps the greatest aspect of this padding is both the peaks and valley texture which goes to the back of the rug, and allows for dirt and grit to fall away from the rug rather than serve as an abrasive touching it. Another good characteristic of this padding is the superior quality rubber which is melted onto the reverse of the padding itself. This rubber is higher quality than most, and from my understanding, carries a ten year warranty which should be covered by the manufacturer. Dura hold is also great for killing noise. expect something in the $2-2.5 per square foot range.
To answer your last question, expensive pads will help your carpet out in ways which most people would not expect. If your carpet is of fairly low quality, you really don't need a high quality padding. However, if the rug is something worth the couple extra bucks, a better quality pad should bring the rug up off the floor just a little bit to protect it from dirt and grit, as well as help keep it in place and protect the longevity. The point of a rug pad should serve dual purpose, cushion the beating a rug takes from everyday traffic, but most importantly, keep the rug in place. An Oriental Rug is not intended to be moved around much. Constant shifting and moving around under furnature and skidding on the floor can only present a danger to those slipping on your rug in home, and also additional unnecessary wear and tear on the rug itself.
I don't have experience with the type of padding on this site sv mentioned, but it's great you're doing the research. Again, DuraHold is the best padding I've encountered for a while now.