you don;t need dummy plugs, even on windows with drivers new than 11.8 I think.
1000W PSU is for sure enough for 3 cards.
I run 5970 and 5850 with 600W PSU.
Ram should be enough for older card types 5XXX 6XXX but I am not quite sure about 7XXX series.
I run those two on 2gb (win7 32)
Running OS from usb reduces power usage, when compared to HDD.
you probably will have to shorten pins on your riser
https://bitcointalk.org/index.php?topic=36061.0this might be helpfull also
https://bitcointalk.org/index.php?topic=117221.msg1256502#msg1256502