New rig with old hardware?

[pccs]

Just a couple of questions before I start putting a rig together.

Building a rig from some old hardware, do you see any problems with this set-up?
I'd like to be able to run 3 cards(hopefully 7970s) in the rig.

MB: Asus Crosshair - 2x16 PCIe and 1x4 PCIe - Should be able to run 3 cards??
      I'll have to run at least one card on a riser, can't get to the x4 PCIe slot.

Processor: AMD 6400+ AM2 - 2.2Ghz
Ram: 2GB

Power supply 1000W or 1250W?

Do I need dummy plugs for each card running Xubuntu 12.04?

Run on USB Drive or should I get a hard drive?

Or should I just use a new motherboard that can accept 4 cards ... do you think building a rig that can run only 3 cards is a waste of money?


Edit: I'm planning on using this for Litecoin as well as bitcoin. I've read that Litecoin uses more ram but I'm currently running one card mining Litecoin and as far as I can tell there is no ram usage to speak of. Maybe I have something configured wrong?

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[pajak666]

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=36061.0
this might be helpfull also
https://bitcointalk.org/index.php?topic=117221.msg1256502#msg1256502