Which is the better deal? 480 or 580?

I can get two gtx 480's for $600 or I can get one 580 for $400. What should I go with?

I'd personally take the GTX580. It will give less performance than the SLI 480s, but then you won't be drawing as much power (can your PSU handle 2 480s?) as well as putting out much less heat in your case (2 GTX480s are going to put out a LOT of heat, and you will need very good case ventilation for that).

+1 gtx 580 could sli later when the price drops .

Probably it will disappear from shops faster then price will drop like it's now with GTX470/480. I wanted to get GTX470 for sli but price for last available models is about 10-15% lower than I paid for my almost 2 yo card. The same I had with HD5850. So even that I'm thinking about SLI/CF I'm always buying new faster single card.
From these 2 options single GTX580 is better idea.

i have 2 gtx480, performance is great, but without watercooling fan noise is unbearable

get a 580.