Requested advice for hardware

I plan to build a system for the company I work for.
Can someone give me some hardware advice.

Requirements:

- Rack based server
- Space for 4 to 6 cards (depends on the number of GPUs)
(What's the max number of GPUs supported ?)

Required information.

- Prefered server case (in connection with cooling and power supply)
- Prefered motherboard
- Prefered graphic cards (it seems that AMD is preferred - price-speed ratio)
- Prefered power supply (one or more)

Intended for all kind of calculations (oclHashcat/cudaHascat)

Please share (recent) building experiences

Thanks in advance