Which CUDA GPU should I buy?

I am new to CUDA and am looking to buy a GPU that will be sufficient for my needs without spending a lot. I will be working on an application that will require graphics rendering as well as other general purpose computations.

What should be my main focus when buying?

• Not. SMs
• Not. CUDA cores
• Core / Shader / Memory Clock
• Memory size
• Memory bus width

How do the above specs affect CUDA performance?