plz have a look on this PC for Local brute force

i am just planning to buy this pc and i would like to request to all LBF experts to advice me to buy this PC and i also wanna know how long this pc will take to calculate one hash.

Intel Core i7-3930K (3.2GHz, 12M Cache, 12x Cores) Overclocked to 4.5GHz Per Core CPU
ASUS P9X79 Pro LGA 2011 Intel X79 SATA 6Gb/s USB 3.0 ATX Motherboard
2 x SAPPHIRE VAPOR-X Radeon HD 7970 3GB OVERCLOCKED EDITION 384bit DDR5 DX11 Graphics Cards In CrossFire
Two Storage Drives
OCZ Vertex 4 128GB SSD Read: 560MB/s Write:430MB/s SATA3 Solid State Drive (OS Drive)
1TB Seagate Barracuda 7200RPM SATA3 6.0Gb/s Hard Drive (Data Drive)
SILVERSTONE Raven RV01 Professional Gaming Case with USB 3.0
Corsair Hydro Series H80i Extreme Performance Liquid CPU Cooler
Corsair AX850 850W 80+ GOLD Fully Modular High Performance Power Supply
Corsair Vengeance 16GB Quad Channel (4x4GB) DDR3 1600MHz High Performance Gaming RAM
24x Dual Layer DVD +/- Writer
Integrated 8 CH HD Sound Card
Gigabit LAN Card - Broadband Ready
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OR
can i ask them to put 2 x HD 7970 6gb cards?
whats the diffrence in between 3 and 6GB cards when it comes to brute force speed?
Regards