1. plenty of people buy phenom II x2 to unlock and plenty of them come crying to this forum when they don't.
2. benchmark reviews will often state how hungry a spec system with that video card will be during a stress test (non real world test) or gaming (real world). if your PSU can cover the stress test with a little head room then you are fine. luckily the AMD 6850 is very power efficient. see below. like obsidian stated a 400w PSU with most of its power on the 12v rail is plenty of power.
CPU: Intel Core i7-920 @ 3.33GHz
Motherboard: Asus Rampage II Extreme
Chipset Drivers: Intel 9.1.1.1015 (Intel)
Hard Disk: OCZ Summit (120GB)
Memory: Patriot Viper DDR3-1333 3 x 2GB (7-7-7-20
http://images.anandtech.com/graphs/graph3987/33245.png
http://images.anandtech.com/graphs/graph3987/33246.png
3. the amount of power needed is determined by all the hardware in the PC that require power.
4. make sure you purchase a quality PSU and not just the cheapest 400-500w you see. currently the antec earthwatt models are on sale and the 500w unit has plenty of power for your build and future upgrades. its a fantastic deal @ $55
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371035&cm_re=antec_500-_-17-371-035-_-Product
budget option at $35
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371033&cm_re=antec_380w-_-17-371-033-_-Product