your friend is wrong, you dont need 750w to run a quad core, you have to factor in all the other hardware youre using and that will decide what psu to get.
you can go with either of the psus that Ste had linked above ^ and save a little money, they will work fine for your rig.
and not a problem on the dual card setup, i was just trying to keep that future option for another upgrade open to you.
in that case, keep the original cpu you chose, the x4 955
you can switch to this mobo instead, its a little cheaper too.
Newegg.com - GIGABYTE GA-MA770T-UD3P AM3 AMD 770 ATX AMD Motherboard - AMD Motherboards
for the gfx card, yes the 5770 supports dx11 while the 4890 is a faster card.
take a look here to give you an idea. Graphics Performance List
i suggested the combo with the 4890 because its just a good deal; $30 off which made it cheaper than the 5770.
if you want the 5770 though, get this combo
Newegg.com - Computer Parts, PC Components, Laptop Computers, Digital Cameras and more!
and you can just go with the original corsair ddr3 ram you picked.