Following previous discussions and theory that 70W is provided from PCIe socket, further 70W from PCIe power connector on the board and whats left per each card is ~30W on second connector I decided it was time to test it in real life environment. I have 5 drives + optical in my Mac Pro, 5th drive is mounted in second optical bay and powered off second molex. Without much hesitation I put second 4870 in MP, plugged in three way molex splitter before optical drive (yes you read it right - one molex to three outs), connected secondary 6 pin to two connectors on splitter, third out to optical and secondary molex to 5th HDD. Booted to windows, enabled crossfire, put Crysis DVD in drive and with a lot of noise, whizzing and whooshing dived into game.
Two and a half hours later of Crysis, half and hour of COD WAW and several random Vantage benchmarks later, gentlemen, I am happy to report the theory was correct, the two cards work, the peak usage in this session was total of 532W, no molex cables become hot and there are no stability issues beyond simple rule of not overclocking any of the two cards beyond spec sheet speeds.
I also made it into neat solution and threaded the splitter though to main compartment, so no cables are being squashed by side panel locks in my box.
So, for 08 Mac Pro - two 4870s, at stock speeds, each powered primary from motherboard, secondary from drive bay - it works, it works well and it works with maximum load - 5 drives with spin down disabled in power options and DVD spinning game disk like nutter.
If I was to be picky - because of the dual slot design the cards don't have much space to breathe, especially card no1. I might try and grab one of the scythe aftermarket coolers just to see if it gives the card more space at some later stage.
