If you're interested in building a new multi-GPU PC, David Tjong, Technical Marketing at AMD, has created a PC build guide
on how to put together several different CrossFireX gaming PCs.
The system packs the following spec:
AMD FX 8150 Processor with Liquid CPU Cooling System
AMD Radeon™ Memory Entertainment Edition 4GB DDR3 x 2
AMD Radeon™ HD 7970 graphics card x 2
ASUS ROG Crosshair V Formula
For the AMD fans it's worth a read to see how AMD themselves do it! But next time, David, how about some HD 7970 DirectCU II
goodies in there too!