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Rick,
I've used many different drivers, I've got 2 6x 280x rigs, 1 6x7950 rig, and now 4 6x 290 rigs actually working. For the 290's I used the 13.11 beta drivers that were just released on January 8th, the 280x uses an earlier 13.11 beta but I'm not sure exactly. It may have even been the r9 launch beta driver, because that was back in November...
So long as you are aware one of the 1x slots will disable it doesn't really matter what you use. You just can't use three and four at the same time. If you are using USB risers your setup is identical to mine, no external jumpers at all...
Which version of linux? I use saucy salamander for the 290's, but the 280x's and 7950's are still on raring since it seems to work. Raring will blackscreen after installing the latest beta driver with 290's, the install log mentions something about a kernel failure. Saucy does not have the same problem apparently. I never could get 13.12 to work at all, not with raring or saucy, though I just tried with 290's. They would all detect just fine, but only one would mine at a time, kinda like they were needing dummy plugs. I don't use any at all with 13.11...