I just received and installed a R9 280x, which I have dedicated to mining. I am using my GTX670 for display and gaming.
Everything I read says I needed a dummy plug for my R9 as I wont have a monitor plugged. I made one from the instructions at overclock. As I am having trouble getting temps and other data show up in GPU Z and cgminer, I just tested without the dummy plug and rebooted. On reboot, cgminer can still use the card for mining.
So are dummy plugs still necessary?
I have yet to use a single dummy plug on any of my windows rigs.