The CPU generally doesn't matter too much as long as it's recent and preferrably has multiple cores. Intel i5s are good and you can probably find a cheaper but basically just as good AMD processor out there.

GPUs are another matter entirely. AMD GPUs often suffer from low performance and people say Catalyst isn't quite as robust as they'd like. However, if open source is close to your heart they are the best option since the radeon driver is simply the most advanced open source driver available. Nvidia side kind of sucks at the open source game but they have the best closed source driver. The Nvidia proprietary "blob" has advanced features including OpenGL 4.5 on their newer cards and superior performance to the Catalyst on equally priced hardware.

It's also worth noting that many game developers will only support Nvidia GPUs. That doesn't necessarily mean their game doesn't work on AMD hardware but it might be unoptimized and in some cases simply refuse to work. Getting support for those games could also be difficult.

So, if you want to go for the absolute gamer setup and accept having a proprietary driver, go for Nvidia. If you are less of a gamer and like open source goodness, go for AMD. Just be aware of the quite sad state of the AMD proprietary driver.

Ok, thank you for your competent answer. I will go with a cheap Nvida now (GTX 960), an will upgrade to AMD in a few years, when (and if) the Open Source Drivers will be up to the task of gaming.

greetings, tom