NVidia CUDA is supported for GPU rendering with NVidia graphics cards. We support graphics cards starting from GTX 2xx (shader model 1.3), however it is recommended to use a GTX 4xx or GTX 5xx card (shader model 2.x), since only those are likely to give good speedup, earlier cards are often slower than just using the CPU. Shader model 1.3 cards also do not support some Cycles features, see below.
Cycles requires recent drivers to be installed, on all operating systems. Be sure to download the Blender version matching your operating system, that is, download 64 bit Blender for 64 bit operating systems.
Rendering using OpenCL is not yet fully supported, but it is being worked on so we can support more graphics cards. Currently only simple clay rendering is supported, due to our full kernel not compiling with the AMD OpenCL compiler.
What renders faster, NVidia or AMD, CUDA or OpenCL?
Currently NVidia with CUDA is rendering faster. There is no fundamental reason why this should be so, we don't use any CUDA specific features, but the compiler appears to be more mature, and can better support big kernels. OpenCL support is still being worked on and has not been optimized as much, because we haven't had the full kernel working yet.