Should I buy a video card now (for example GTX 970) or should I wait for NVIDIA's Pascal cards?
I have saved some money over the past few months to buy a new PC for 3D modeling and game design.
That really won't run anything well...maybe Blender and you'll need a decent power supply for a 970....I suggest a new build or buy used...look for an I7, motherboard that supports 16 GB RAM (preferably 32!)...you can find used HP Z600's or Lenovo M83's for under $500 that will get you started....then go for a Pascal!
The current Nvidia cards got massive problems with asynchronous processing in DirectX12. Wait for the next series or until they fix this issue (dunno if it can actually be fixed via software alone).
if you can't use the your current time then you might want to wait and get the increased power for the same money