Q2 wrote:But where can I buy a PC with that configuration which is as reliable? I checked HP, Dell and BoXX and came up with even higher prices for a similar configuration.... And I am not sure I want an oveclocked computer running 24/7. How reliable is that?
Yes, HP, Dell and Boxx are maybe not much cheaper and especially Boxx has really funny prices... And Dell and HP still have no builds based on the new Xeon E5 V2. And only very limited configuration options (i think at least in this part they have managed to imitate apple)
I think, if you could build it yourself you could end somewhere around 3800€, with one Xeon E5 V2 12-core 2,7GHz, 32GB, 256GB SSD and TWO AMD FirePro W7000 (if you want a FirePro - and if you need two of them)
If you really want a PC instead of a Mac and you don't want/can build it yourself, you could look for a (local) pc shop that sells custom build systems and provides also support.
Maybe something like this:
http://www.mifcom.de/17/workstation.html (only as example - i don't know them, this was only a google result for shops in Munich that provide configurable workstations and service plans)
And concerning overclocking an reliability... i have overclocked all of my machines since maybe 15 years now and currently i'm running 10 overclocked systems. I don't see any problems here - if it is done right and in a reasonable range.