You'll want to be a little careful when buying a tarp from a retail outlet like REI or Campmor. They sell great tarps but they also sell less good tarps. For instance at REI they sell a tarp called a "Coated Tarp" and another called a "UL tarp". The UL tarp is made of silnylon, which is what you want. The other is polyurethane coated nylon which, while waterproof, is heavier than silnylon.

Campmor is even more complicated because they sell two tarps of the same size from the same manufacturer but only one is really for backpacking. The Equinox Ultralight Tarp (which is the same as the one REI calls the UL tarp) is the good one. The other is the Equinox Coated Nylon tarp and is almost twice the weight of the Ultralight Tarp.

In both cases the more expensive one is the one you want I'm afraid.