The Denoiser will first try to use CUDA, if it doesn't find a compatible card, it will fallback to OpenCL GPU (usually with AMD and Intel cards). If the image doesn't fit in the graphics card memory, you can then use OpenCL with CPU (which will use RAM memory).
Maxwell is compiled for 64-bitONLY. It requires a 64-bit machine and a 64-bit operating system. Plugins are available only for 64-bit builds of their respective host platforms.