I've just bought a new monitor. It's the Samsung C24F390F, which I wanted to use as my second monitor. The problem is that my PC just can't detect it. Tried different connections, HDMI-DVI, HDMI + DVI Adapter and a normal HDMI-cable but none of them worked for me.
I connected the monitor to the Mac of a friend of mine and there it worked without any problems, so it seems to be a problem between the R9 270X and the Samsung monitor.