Why a 580gtx though, I'm more pro AMD but have no issue with people buying Nvidia. The 570gtx is just SO much better value.
I can't say what the price difference is in Norway but a 570gtx is £233 on ocuk, and the cheapest 580gtx is £360, thats over 50% higher cost, for what amounts to at best, 15% performance, its just not worth it.
When new cards launch aswell AMD or Nvidia cards, the current gen drop in value badly, if you wanted to sell on and buy a new card, the 580gtx WILL devalue far more than the 570gtx.
Lets say you buy a 580gtx, for £360, the new cards launch, suddenly no one will buy a second hand one for over £180, and a new theoretical 680gtx costs you £360 brand new, it would cost you basically £180 to upgrade to a new gen card should you want to.
If you buy a 570gtx now for £230, sell that for £140-150 and buy a new 670gtx for for £270, it will cost you £100 to upgrade to a new gen should you want to.
But factor in the initial saving and you've got a 570gtx, and then moved to a 670gtx, and still spent LESS than you would on a single 580gtx.
The highest end cards are just awful value, the 570gtx at £230 is a really good card and very good value, the 6950 is a good value card, the 6970/580gtx are pretty rubbish value in comparison.
In 99% of cases you really won't notice the difference between a 570 and 580gtx, nor a 6950 and 6970.