A quality 650W or 750W PSU is plenty for your system with up to a single GTX680. You could even get by with a 550Watt but it's best to get a 650W/750W.
That totally depends on your budget.
I tend to lean towards NVidia GTX670/680 for high-end gaming machines. I have the Asus GTX680 TOP card which is about $540 and three slots. There are also other good 670/680 cards from Gigabyte and others. Always get one with a nice non-reference cooler and possibly read a review on the card.
The following is on sale in the US and reasonably priced (or use as examples):
Graphics Card: http://us.ncix.com/products/?sku=71107&vpn=GV-N670OC-2GD&manufacture=Gigabyte&promoid=1295
Asus has some nice cards, Sapphire has nice AMD cards (HD7000 series), and Antec and others have good Power Supplies.
I don't want to get bogged down with AMD/NVidia etc so if you have any specific questions just ask.