I know this question has been asked many times, but frankly it has been months sine those threads and also i couldn't find any threads regarding single monitor.
Im putting together a new rig with 2600k and 8 gb 1600 ram. i also wanted to add in a gtx580 ( since 590 is always out of stock) and was wondering is 3 gb a bit over kill? i mean if i want to max out all settings in high end games for a single monitor (1920*1080 or somewhere around that) will i need 3gb or 1.5 will do just fine?
1.5 should be fine. I've seen articles on slightly lower-end cards that saw no difference between 1 and 2 gigs, so 1.5 and 3 for this beast should be a similar circumstance.
It depends what you mean by maxing. To me, maxing means a consistent 60fps, and a single 580 won't always provide that. If you're fine with, say, 50fps on Crysis 2, then go for it.
I was in the same situation a while back, and just realized after purchasing my cards. More Vram normally benefits multi display gaming. For example, if you are using a 1.5 GB card with all settings maxed for 3 monitors, you will be good, but there may be some performance degradation, compared to 3 GB and 3 way setup, no worries.
However, for just one 1080p monitor, the 1.5 GB is more than enough headroom for future demands on games, and you are good. Oh, and if in the future you are planning on getting a SLI setup, you are going to end up paying a little more for the 580 3GB card and because you are not utilizing 3 monitors, imo the 3GB is not worth it.