I was hoping to have better news by now. I've come to the conclusion that the hardware is just old and horrible. That being said I'm not updating it yet.
The OpenBSD install went well, but that is about where it ends. HDMI audio was a no-go. Azalia found no usable codecs. The graphics card which is build on the old Athlon II the HD 4200, uses the R600 driver. OpenBSD couldn't load the drm for the accelerated graphics.
So out of the 2 FreeBSD worked better. Part of me is debating trying FreeBSD 11 to see if any of the new stuff will help with some issues. But ultimately I may have to go with a Linux for this one. And maybe the goal to switch away from Windows out weighs the goal to switch to totally using BSD variants instead of Linux, that was mainly based on my servers anyways.