If you buy either card linked expect to be modding if there are any hot spots why because most out there that have cards fail on them often didn't bother to mod nor maintain their cards. Go with EVGA if you want a good warranty or possible step up program.
It's called the law of diminishing returns. Ya want the best, it costs. If ya have a lot of disposable income, it's not worth a 2nd thought. If not, buy within ya budget. But look at the whole picture..... looking at the relative increase in cost of the card (580 costs 42% more for 13% increase in performance) is looking at it the wrong way. It's not your GFX card that goes faster....it's your entire system that goes faster. So on a $1500 system for example, getting a 13% increase in perfermance for a 10% increase in price seems like a fair investment.
Another way to look at it is twin 570's over a single 580.....looking at relative to GFX card cost alone, it's a 41.72% increase in performance for a 42 % increase in price......or a 12% increase in system cost
To my mind, a 570 in a full tower with an 850 watt PSU (XFX Black Edition 850 / Antec CP-850 or Corsair HX850) is the way to go. One 570 now, and add a 2nd come XMas 2011 or 2012
If you are paying cash, then the GTX570 will enable you to have some extra left over. If you are paying by credit, the GTX580 will only amount to 2 or 3 extra months of payments (one way to look at it). There are games out there where you will want every bit of additional horsepower available in the GTX580.
It's hard to go wrong with manufacturers; the stock/reference boards are very nice and will not vary at all from brand to brand. If you are looking at non-reference boards with custom cooling, then those are worth doing some additional research.
PNY is an American company that offers a lifetime warranty and good value accessories in the box (a full mini-HDMI to HDMI cable worth about $50, for example). So no problem, if you decide to go with them.
The GTX590 will not change any prices of the other cards, I think. Almost all GTX580's are now $499 across the board, whereas they started out at around $520. The GTX570's are $349, unless you go for a custom model.