I agree with morphine.
The 470 in SLI will perform about 10-15% better in most situations but it is power hungry. You can easily hit 550-650 Watts of total power use, and the cards will easily go over 90C in SLI configuration. In comparison with a single Geforce 570, your total computer power usage will be about 230-330 Watts at full GPU load depending on your computer configuration. It also runs about 15C cooler and with about 10-15 Db less sound than the 470 SLI well.