First of all, thank you very much! Second, I used Cinema 4D with the standard rendering engine. I used a MacBook Pro with 512 MB (not sure what the chip is called) Graphics card and 4 GB RAM, but now I also have my own "gaming" PC that I now use to render stuff. This PC has an 8 GB Quad Core i5 and 1.5 GB graphics card.
It depends. If you have a CUDA based GPU, you will benfit from such renderers as iRay or VRay RT (newest version only). If your GPU is old, the rendering won't be much faster (depends on the number of GPU CUDA Cores and the clock rates). Not sure about Octane, Indigo, FryRender or Arion. Any other GPU will only accelerate Quicksilver and VRay RT (older versions. GPU has to support OpenCL). It's tricky.
Any other GPU will only accelerate Quicksilver and VRay RT (older versions. GPU has to support OpenCL). It's tricky.