here is a slight chance that your existing graphic card supports CUDA.
You cannot buy any GPU, install it and expect it to work. Besides the obvious:
motherboard compatibility, the most restrictive reason is capacity of power
module of your PC. Most home computers are designed with energy efficiency in
mind. The power module probably has only a slight excess capacity over
requirements of hardware configuration.
Most importantly, do not do it without assistance. Go to Nvidia website and
ask for help.