Tomorrow when you move one of the two HD5870s to the lowest PCIex16 slot to test out possible conflict with all the headers, I suggest you to move the upper card.
With this, you will also be able to simulate the future tri-fire situation where the 2 lower cards have no empty slot in between them. See pic below. The red arrow points to that narrow gap. Run a few game benchmarks to check the temperature of the 2 cards, especially the upper one (ie the one 'middle 5870' in the pic below).
IMHO, this will be the biggest challenge that you will face when you do a tri-fire setup. The middle HD5870 will run much hotter. Having said this, it might not cause BSODs but I expect the middle card to hit , at least, high 80's to low 90's C when gaming.
And, since you say that your Phantom currently is barely offering enough air for your current pair, 3 cards will be even more 'air-demanding', particularly to the middle card.
IMHO, you will need a case with strong air supply from the side pointing at that gap (and preferably from the case front intake as well) to provide good cooling. That is, it is not just a case with a side panel fan. Obviously a case with a side panel fan pointing at the CPU area is not going to help out in this situation. The fan(s) must point to that specific area. This is a feature of a case that you should go after, in addition to the 8th expansion slot issue.Edited by windfire - 5/16/11 at 1:21am