to turn the 290X's fan down from jet-taking-off to kitchen-stove-fan, so I can hear myself think. This brings hash rate down to 520 or so. And then when I go away
aticonfig --odsc 1000,850
for normal speed of 600-640 MH/s.
Try putting a fan (at least 12cm) right at r290(x) side (near the card fan, 90 degrees to the mainboard), blowing in the card direction. Helped me reducing the noise greatly (it also reduced power consumption, the r290 fan sucks a lot of electricity when turning fast).
Not enough room to do that on my rig. It's not on a tabletop, rather in a desktop tower case. The 290X is on the first (physically, the upper) pci-e slot and the 260X sits below it on the second pci-e slot. Out from the end of the 290X, the hard disk carrier occupies the rest of the space to the front of the case.
I do have an 80 mm fan blowing across the 260X, parallel to the motherboard. That cools both cards down. Both cards run at ~80 C when the 290X's fan is on high, and at 75 C when the 290X's fan is on low. I suppose when I have time I could switch the cards' positions, but then the airflow to the 290X's fan would be restricted by the limited space between the fan intake and the floor of the case.
If I had to build a fresh rig I'd probably go with three 260X's rather than one 290X. Same hashing power but much quieter. At Fry's, three 260X's sell for $375+ tax after rebate. A single 290X nowadays sells for $619 +tax +shipping. For the $244 difference you can buy an MSI Z87-G43 mb, a cheap Celeron cpu, and a 2 GB ram stick, then run it on Puppy Linux booted from a USB thumb drive. Add $99 for a Corsair RM 650 power supply. Then use your first two weeks' income to buy a second 260X for the remaining slot, and you'll end up with the same 605 MH/s hashing power as one 260X plus this frickin' jet aircraft 290X is giving me.