GDDR5 vs. DDR3 for learning GPU programming on a limited budget?
If your goal is to learn GPU programming, it doesn't matter at all whether you have DDR3 or GDDR5 memory. The way you program it isn't going to change based on how fast the memory is. It will affect performance, but if that's not a primary consideration for you, then you don't need to worry about it.

Do make sure that you get a GPU that supports the latest graphics APIs (DX12 and OpenGL 4.5). I think that's essentially all GPUs made in the past 5 years or so.