...and then try and buy the video cards on the cheap second hand.
Good luck with that, as second hand 5970s are going for more than when they were new in some places.
I didn't want to spend large amounts on a 5970, so I got a 5870 from OcUK for a reasonable price:http://www.overclockers.co.uk/showproduct.php?prodid=GX-147-XF&groupid=701&catid=56&subcat=1711
Its running at stock volts, 950 Core and 300 Mem. I originally flashed the BIOS with the ASUS BIOS for higher voltage, but it just got hotter and didn't overclock any better so I flashed it back. I get 360 MHs out of it and it runs at about 270 Watts with the monitor turned off.
When I get my act together I'll try and build a pure mining PC with a more power efficient setup, as my current PC is a Q9650 with 2 sound cards, 3 hard drives, etc, etc...