It is my understanding that the R9 290 uses the built-in drivers built into OS X i.e. Yosemite. So after the Mac has booted in to Yosemite the card should 'just work'.
What may be an issue for you is being able to use pre-boot functions like holding down the Option key to select a boot drive, or other keys for other functions all of which happen before the Mac has finished booting. Typically these only work if the video card has a 'proper' Mac compatible firmware loaded. A standard retail R9 290 card will not have Mac compatible firmware.
The lack of the Mac firmware will not prevent the card working after the Mac has finished booting, it will just as mentioned mean you cannot see the boot process and hence cannot use pre-boot key strokes like Option.
It is possible to 'flash' the firmware in an R9 290 card to add Mac firmware to resolve this. This is not officially supported by Apple or AMD but it does work and many people have successfully done this. You will have to Google search the solution if you want to do this, however it will require either fitting the card temporarily in a real Windows PC, or using Windows via Boot Camp. Parallels and VMware Fusion or VirtualBox will not work for this. (The software to flash the card runs in Windows.)
Note: You can change which drive to boot from i.e. from the Windows Boot Camp drive back to the Mac drive using the Boot Camp control panel in Windows if you have installed Apple's Boot Camp drivers. Likewise you can use Startup Disk in System Preferences on the Mac to switch to booting from Windows. This does not require using the Option key trick.
