Word.

http://mark.zoomcities.com/images/gfx/GFXpowerchartbybrandgen.png

http://www.bit-tech.net/hardware/2010/12/16/ati-radeon-hd-6950-review/10

Less power consumption 6950 and 5850, than 7***. There will be a bit more heat, but there are solutions for that.

7* has probably been done before. Here is topic about 6*:
https://bitcointalk.org/index.php?topic=142241.0

BIOS over clocking, yeah.

I might just buy the motherboard I linked, but start with 5 gpu.

Less than 1k is a small budget, i think.

PSU:

1500 W should be more than enough and there are such devices.

Putting all together is that worries me : D

EDIT: thanks for the answers btw.