be aware a 5970 + 5870 can be safely run on a 850w psu but 2 overclocked 470s can hit 1000w peak draw 480s can touch 1100watts or more think of it this way 2 480s use 320w each at max load so 640w draw but its not 100 620 watts is closer to the actually gpu only load where as 3 5870s or a 5970 + 5870 will hit the same power draw roughly
cant find it right now but bodar did a review on crossfire from 1 to 2 to 3 to 4 gpus using 5870s
found it
http://www.youtube.com/user/TheBodar.../0/lCj4r2Gskqw
intresting look is Far Cry 2
1 gpu = 65
2 gpus = 129
3 gpus = 214
4 gpus = 236
where as the gtx 480 seems to be
1 gpu = 84
2 gpu = 151
3 gpus = 174 ( most likely cpu bottleneck here
in crysis 3 5870s get around 70fps
3 480s hit 87 and 2 hit 83
from the looks 3 5870s use less power and are faster then 2 480s but ymmv
for the most part tho 2 gtx 460s or 2 5850s max everything
and as far as the Metro 2033 thing ALL gpus struggle with that game i dont care if its nvidia or ati turn on the DOF settings and the game chugs on stock 5850s i average 50 fps at 1920x1200 with DOF off but DX11 very high with tesselation on
altho if you have the PCIE slots i suggest 3 5850s due to there overclocking and the fact that the tesselator in the 5850s is the same as the 5870 means as DX11 grows the gap between both will diminish and an overclocked 5850 is 3% slower on average roughly 1 fps in crysis etc if you get reference models they max out same as the 5870 and some have had luck flashing the 5850 with 5870 bios giving higher voltages allowing easy reach of higher clocks among other things just food for thought
again no matter what setup you pick your gonna have insane performance