The B2 is far superior to the CX600. The CX series is designed for light duty gaming, not the kind of punishment a r9 290x can dish out.
It'll run it, barely, if all you use it for is net surfing or other light duty graphics, but you run a real chance of burning it up under heavy graphics demands, and since the main power draw will be the gpu at that time, a good chance of spiking the gpu as well,possibly ruining that too.
Personally, I wouldn't chance it, but it's your stuff.