I just purchased 2x 5870 Mac graphic cards (Apple) and plan on installing them in my 2009 Mac Pro Quad Core. This is part of my upgrade plan, installing two of these along with a W3680 Hex. Two years ago, someone uploaded a video demonstrating that two 5870 cards can be installed using 6pin->2x6pin splitters and crossing them to even the load on the cards. [LINK] A year later the same person stated that "No problems so far. My Mac Pro is on between 12-14 hours per day every day and power supply fan just idols along even when under heavy load." Another year has gone by and this person is still using the 2x5870's in their 12 Core Mac Pro. So far, I've used a single GTX670 off both power connectors in my QC Mac Pro. I moved away from it due to the lack of EFI. It worked just fine, AFAIK that you can't use a GTX580 / 590 and many other cards without an AUX power supply. With the two 5870's I've ordered, people have stated that it is unsafe due to the wattage that the PSU can supply and the tracers on the board. In the case of the aforementioned person, they have been using the configuration for two years at "12-14 hours a day". So does this mean that the theories are false and it's SAFE to use dual 5870's in the Mac Pro? Calculation: 5870 POWER: 188W 5870 POWER: 188W TOTAL POWER: 376W PCI-E LANE POWER: 75W PCI-E LANE POWER: 75W 6PIN POWER: 75W 6PIN POWER: 75W TOTAL POWER: 300W It appears there is a lack of power that can be supplied and it's not likely that the cards draw MAX power all the time. The biggest question is about the voltage. I've read stories written by fellow members about using two cards, many of those cards include the 2x 4870x2 and 2x GTX580's. Those cards pull almost 500W total. Following this [LINK] gets you to a listing where the seller has sold 152 splitter cables. Assuming that only 1/10th of the people are using high power cards, it's possible that this setup "just works". The question is still about copper. To avoid damage, one could route a SATA -> Molex as well as 2x 6pin cables out the back of one of the PCI-E slots and plug it into an external PSU. That's the cleanest way of getting an AUX PSU working. I will definitely try with and without the ext. PSU once I receive the 5870s.