The R9 280X pulls around 850 kH/s of scrypt hashing on optimal settings without overclocking, and that's not half-bad. However, if you're serious about making an effort with this you should try locating an HD7990 instead. It costs just marginally more than the 290X but does around 1350 kH/s before overclocking, and can reach 1500 kH/s with a tiny amount of overclocking - assuming you can manage to cool it properly so it doesn't throttle down from the heat.