first timer, trying to setup a rig, recommendations regarding PSU and GPUs types

[zxyzxy]

Hello, i want to try mining, one computer, max 2-3 cards..
i was thinking about radeon 5970, 6990, 7990, alternatively 7970

lets try combinations like these for starters, these would suit me.

2x 7990 (dont know if i can get my hands on these yet)
3x 5970
1x 6990 1x5970
1x 6990 2x5970
2x 5970

my question is, what should be the optimal PSU for these setups?
from what i understand, even if i plan to use only two cards, i should buy a 1200W unit?
also, do i need a water cooling or external fan if this rig will mine 24/7?

additional relevant question, is there a page online where i can browse motherboards sorted by types of cards they support? i would like to invest into a board with at least 3 PCIE
 slots (does it matter if they are 8 or 16? is there a reduction for this, if so and the card is connected throught a reduction, will the hash rate be affected? i never used such "high-end" cards)

really any info regarding this matter will be appreciated, i am very interested to start my own rig!

[MashRinx]

Some answers:

- I used NewEgg.com to narrow down some MB choices and also did some searching on this forum as well.  There are all kinds of different boards being used for mining, but I saw a recommendation of the ASRock 970 Extreme 4 and bought quite a few of them ay $99 at the local Micro Center.  They have been solid boards and if you mine with 2 cards, you can space them out to where you don't really need PCIx Extenders if you don't want to use them.  I am looking at a machine right now where this is the case and temps are fine.  Any board with 3 or 4 16x PCIx slots is probably similar.

- The PCIe Extenders I have used are 16x to 16x  and 1x to 16x.  I have not used the 8x, but I can say that there is no reduction in hashing rates using a 1x to 16x extender.

- I have a machine currently mining with 2 x 7970s with a PCIe HD, cheap AMD CPU and 1 x 2 gig stick of RAM and I am pulling about 670 watts.  That could probably be lowered under-volting the GPU memory, but I haven't tried.

- none of my mining machines are in cases.  Don't need them to look pretty, and want more free airflow.  I am not water-cooling anything and do have fans behind most machines, but not all. 

Hope this helps...

[glendall]

Mash, if it's not too much trouble could you link to the PCI Extenders you are using? Are they pretty much like these:

http://www.amazon.com/gp/product/B002M1DHJG?tag=at055-20

I haven't used extenders before but think I will try them out.

Zxy-=>  1200w is overkill for 2 cards IMHO.  Two cards can run on quality 750W psu.  1200w would be more suitable for 3 cards.

I'd also recommend just building a 2 card system first instead of a 3 card one. You'll have the ability to go to 3 cards but it'll much easier to put the 2 card machine together at first.

For GPU's I would  not go with the 5000 series unless you found a really good deal on used cards. For price to performance you'll want to go with newer generations.

I'd also recommend sticking with video cards of the same series. As you are just starting out, it will be easier to determine any problems, and see your performance levels, if you are using the same cards.  It just gets a bit more complicated if you are using a mix-and-match.