Newb help with hardware selection 3 x 7950

[mpj76a]

Hello,

Started mining with the laptop to figure out some basics and now I want more speed.  I was thinking about the following:

Case - Cooler Master LAN Box
GPU's - 3 x 7950
Power - Cooler Master 1200W gold
Motherboard - Asus M5A99FX Pro R2.0

I didn't include memory or CPU, but they're decent (8GB, AMD 8 core of some sort).

I'm pretty sure I could have gone lower on the CPU and memory but I'm just keeping it flexible for other stuff.
After reading a few posts I think the power source might be overkill.  If so, what should I get instead?
I see that the motherboard supports 2 x PCIe 2.0 x 16 slots (dual @x16/x16 speed) and 2 x PCIe 2.0 x 16 slots (x4 speed).  Shouldn't the motherboard have 3 x PCIe 2.0 x 16 (x16 speed)?  If so, can you recommend one?

The above was recommended by the sales guy based on the fact that I want to run the 3 x 7950's.  They are specifically XFX Radeon HD 7950 Double D Video Card - 3072MB, GDDR5, PCI Express 3.0 (x16).  Is this good?

Thanks again!

edited to add (x16 speed) clarification

[mandroid]

I am building a rig tomorrow (parts ordered) that will have 3x 7950s. I suggest reading through this thread as another newbie got a lot of help from a forum member: https://bitcointalk.org/index.php?topic=171081.0

The most important note is that the LAN BOX cannot fit three cards without modifying the case. The thread also contains good info about selecting a motherboard, cpu, etc that will cheaply yet successfully house the three cards. All up my rig is going to cost me AUD$1800 (roughly USD$1900). As a last note, I think that PSU is overkill for system + 3 x 7950, you need around 800W (200 per card, 200 for system worst case scenario).

Here is my rig as the order stands now.

Order Details
1       D-Link DWA-125 Wireless Lan 150 USB Adapter   
1       Microsoft Windows 7 Home Premium 64bit OEM   
1       Kingston 8G(2x4G) DDR3 1600MHz CL9 HyperX       
3       Gigabyte ATI HD7950  3GB  GDDR5         
2       BitFenix 200mm Spectre PRO Series Fan Black, High Pressure/CFM Desig         
1       Western Digital 500GB 64M IntelliPower SATA3 Caviar Green  WD5000AZRX     
1       CoolerMaster RC-922XM-KKN2 HAF Black  USB3.0         
1       Corsair HX850v2 850W ATX Power Supply, 80 PLUS GOLD             
1       ASRock 970_EXTREME4.AM3+ AMD970 4*DDR3 SATA3 USB3.0 3*PCIEx16 RAID             
1       AMD FX-4130 4-Core 3.8GHz  Socket AM3+ 125W BE PI             

[mandroid]

Two more notes I picked up from the thread I linked:

-the CPU literally only has to be the cheapest to fit your motherboard, it is NOT needed.
-RAM for BTC mining requires 2GB, for LTC mining 8GB (worst case)
-cooling is VERY important, make sure the version of the 7950s you are selecting have HEAPS of cooling (the gigabyte model I selected has three fans) and get some big, high-rate, NON-silent fans into the case

[Timzim103]

For three 7950s, I'd say 800w isn't overkill. I measured 675~ on my rig. It has only two 7950s at -i 9. Perhaps get a kill-a-watt to monitor this...

[mpj76a]

Thanks for the answers so far.  Was I right about the the fact that my proposed card doesn't support the third GPU?  I'm new to these specs, but I read it as two good, the third not so good.