If it uses the reference GTX 570 design with four power phases, do not over-volt it. If you do, it will blow one of the power regulation chips, as the power design of the reference cards sucks.
Get a card with a non-reference design, like the Palit GTX 570 Sonic or Sonic Platinum. They have six phases for the GPU and two phases for the RAM. There are others that have five phases, and those are pretty good too.