What is a CUDA graphics card?

And why does Blender Cycles require it all of a sudden?

What exactly do you mean by “all of a sudden”? GPU rendering with Cycles did always require a CUDA-compatible graphics card.

In short, CUDA is nVidia’s proprietary programming platform for parallel computing applications. It is supported by almost every nVidia GPU made since about 2007, although older models might not support all available functions. You’re on the safe side with GPUs from the Fermi, Kepler and Maxwell families.

Just be careful with Maxwell as they are slow(er) with 2.7+ for now.

You do NOT need a CUDA graphics card to use cycles.

Only if you want full GPU rendering support. CPU rendering will still work with any AMD or Intel CPU.

I am finding it slow at the minute with my cpu, I am using an intel quad core processor . I also have quite low budget grphics card which is nvidia 8400
theoption is not available to run by my cpu to run, i have dowloaded the latest drivers .

You graphics card does not meet the minimum requirements. See http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/GPU_Rendering
Before you ask, you cannot fix it by updating drivers etc.

Two take-aways:

  • Not all graphics cards are equal, even if they are CUDA. (Some of them frankly suck.)
  • If you do have a CUDA card and want to use it with Cycles, you’re going to need to have the proper software driver installed, external to Blender itself.