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.