The 2x GTX570's will cost ~$700. A single GTX580, like I have, will destroy any game at any resolution at any level of maximum details. I would not get two GTX480's, as they get hot and loud. The 570/580's have very good heat and noise signatures in single and dual card configurations. In fact, when you add a second card, they will only register a minimal increase in temperature and noise level. Since their excellent vapor chamber cooler vents heat outside the case, the impact on your system/case temperatures is minimal.
I see your two options as:
-Get 1 GTX580
-Get 1 GTX570 and add a second later on when you can afford the extra expense (for that matter, you might be able to afford a second 580 later on, if you decide to go that route)http://www.guru3d.com/article/geforce-gtx-580-review/http://www.guru3d.com/article/geforce-gtx-570-sli-review/http://www.guru3d.com/article/geforce-gtx-580-sli-review/