Some parameters of these gpus are as follows:
model memory cores price ($)
Geforce Titan 6G 2688 600~800 (amazon)
Geforce TitanX 12G 3072 1100 (@Xi,amazon)
Tesla K20 5G 2496 1800 (amazon)
Tesla K40 12G 2880 3000 (amazon)
Tesla K80 24G 4992 4000+ (amazon)
After plugging the GPU onto the motherborad, always choose the right driver for it: http://www.nvidia.com/Download/Find.aspx
After installing the driver, install the cuda toolkit,
http://insiderattack.blogspot.com/2014/12/installing-cuda-toolkit-65-in-ubuntu.html
Sometimes, you may want an additional cudnn package for convolutional neural network. Download cudnn library, unpack and copy its files to appropriate directories.
sudo cp dir_of_cudnn/lib* /usr/local/cuda/lib64
sudo cp dir_of_cudnn/cudnn.h /usr/local/cuda/include
Detailed guidance of other hardware can be found at
http://timdettmers.com/2015/03/09/deep-learning-hardware-guide/