Can a R9 290X mine?
With Radeon R9 290/390 GPUs you can currently get around 29–30 MHS in terms of hashrate mining Ethereum (ETH) with about 150–200W of power usage per video card (with optimized/reduced power usage).
Is the R9 280X good for mining?
The Radeon R9 280X can manage around 360 KH/s under Windows and 370 KH/s mining VTC under Linux with the default out of the box settings (925 MHz GPU and 1375 MHz VRAM), but overclocking it a bit to 1050 MHz for the GPU and 1500 MHz for the memory can give you a bit over 400 KH/s as hashrate.