Hi,
You are fine with this power supply. It has a single rail design with 70.8A which is enough for a CrossFire configuration with the 290 cards.
OUTPUT
| Rail | +3.3V | +5V | +12V | +5Vsb | -12V |
| Max output | 24A | 24A | 70.8A | 3.0A | 0.5A |
| 849.6W | |||||
| 120W | 849.6W | 15W | 6W | ||
| Total | 850W @ +50C | ||||
Regards