Advice on system build and power supply?

Sandy Bridge chipsets P67/H67 aren't fixed at the moment, so you have to wait.

12GB ram (2x4GB + 2x2GB) is too much (btw, LGA 1155 mobo's aren't triple channel, only dual channel), 2x4GB DDR3 1600 is plenty.

the HD 5850 will be discontinued pretty soon. instead i would suggest the HD 6870 that has about the same price.

an adequate psu for this computer is 650W (premium brand, Corsair or Antec), e.g. the Corsair CMPSU-650HX

added

the i7-950 is a different story. this cpu requires an X58 motherboard that has triple channel ram, so you can get 3x4GB. (on a motherboard with dual channel ram 12GB is a bit odd, therefore it's better to get 2x2, 2x4 or even 4x4).

try this power calculator http://www.coolermaster.com/support/psu_calculator... It will give you a reasonable idea of what you will need

I would say minimum 500watt PSU with 2 Pcie 6 pin power cables and definately a Brand name PSU

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