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=171081.0The 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