Reddit mentions of PCIe 1x to 16x Powered Flexible Riser Extender Cable with Molex (Bitcoin Mining / Gaming)

These are lower quality risers. I recommend something like this instead

http://www.amazon.com/Powered-Flexible-Extender-Bitcoin-Mining/dp/B00EX627A8/ref=sr_1_6?ie=UTF8&qid=1394213776&sr=8-6&keywords=usb+riser

http://www.amazon.com/Express-Powered-Extender-Litecoin-Bitcoin/dp/B00HH1MLGQ/ref=sr_1_3?ie=UTF8&qid=1394213776&sr=8-3&keywords=usb+riser

PCPartPicker part list / Price breakdown by merchant / Benchmarks

Type|Item|Price

----|:----|:----
CPU | Intel Celeron G1820 2.7GHz Dual-Core Processor | $48.23 @ Mwave
Memory | Crucial Ballistix Sport XT 4GB (1 x 4GB) DDR3-1600 Memory | $39.95 @ Amazon
Storage | Sandisk ReadyCache 32GB 2.5" Solid State Disk | $39.27 @ Amazon
Video Card | Asus Radeon R9 280X 3GB Video Card | $486.13 @ Newegg
Power Supply | LEPA G Series 1600W 80+ Gold Certified Fully-Modular ATX Power Supply | $297.98 @ SuperBiiz
Other| ASRock H81 PRO BTC| $99.99
| | Total
| Prices include shipping, taxes, and discounts when available. | $1011.55
| Generated by PCPartPicker 2014-03-11 20:48 EDT-0400 |
That and 4 of these.(http://www.amazon.com/Powered-Flexible-Extender-Bitcoin-Mining/dp/B00EX627A8/ref=pd_cp_pc_1)
With this and those 6850s and 7950 you should get around 1.8-2 Mh/s.
EDIT: Don't use a case, open air is best for mining. A lot of people use milk crates.

If you are concerned about PCI-E power draw, just get a powered PCI riser cable. You can plug a molex into it and it'll draw power from there rather than the motherboard.

Heres an example
https://www.amazon.com/Powered-Flexible-Extender-Bitcoin-Mining/dp/B00EX627A8/ref=sr_1_5?s=pc&ie=UTF8&qid=1467321262&sr=1-5&keywords=powered+riser