I voted best answer too soon. I found some relevant info about gtx 580s.
Here is the link to some charts for a 3gb vs 1.5gb 580 at 2560x1600.http://www.fudzilla.com/reviews/item/21800-gainward-gtx-580-3gb-phantom-tested/21800-gainward-gtx-580-3gb-phantom-tested?start=5
As you can see in all the charts there is very little improvement from having 3gb of vram. I don't know if newer games will benefit from more VRAM but from those charts 1.5gb seems to be fine.
That being said I also found something else interesting. MSI will be releasing a gtx 580 3GB lightning model soon. And seeing as I am still waiting for a few releases for my build, I think I will grab two of those just in case. They don't look better aesthetically than the normal 580 lightnings though.http://news.softpedia.com/news/MSI-s-3GB-Packing-GTX-580-Lightning-Xtreme-Edition-Gets-Pictured-200816.shtml
Edit: There are also rumors of an Asus Ares 595, which is supposed to be based on the same cooler as the other one they released for ATI. From what I recall, the last Ares was way overpriced and did not perform up to the price tag.