I have the R9 280x on one of my machines. It's a pretty decent kit. Should be enough to run most of the Rift demos out now (since many of them are w/ Unity and the engine is forgiving). Personally if I was buying again I would get the R9 290, since it's a much better card for only $100 more. It also competes favorably to much more expensive cards from Nvidia.
However, if you only have $300 to spend, the 280x is not a bad choice.