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 point a fan at the side of the tower case. So the fan is parallel with the motherboard blowing fresh outside air at the top of the cards and their fans suck that in. I think that might be what he meant, putting an extra fan blowing in to the existing card's fan doesn't make any sense.