If you want to upgrade any pc to designer user what is the most important things in the pc ?

Hello, this is a general question, so if possible, add more details about the hardware of the existing computer so others will be able to help you. 

But in general , design softwares need high speed processors , I recommend i7 because it has8 threads and this helps software perform faster. If i7 is not an option , then go to i5, this one has2 cores (4 threads). If you have bigger budget, you can go to Xeon processors , what gives your PC lots of raw power.

The second thing you should look at is the RAM ,8 GB is OK .

Those two factors (CPU and RAM) are the most important and they have clear effect when you run the design software. By the way, they depend on the type of the motherboard, you have to check this first before you buy anything. Many motherboards can't be upgraded to host such hardware. The power supply too.

The third thing you should look at is the graphics card. This one is hard to explain in two sentences but the decisive factor is the "bandwidth speed". nVidia and AMD have a wide selection of cards to choose from , but in general , what works for games IS NOT necessarily alright for CAD, CAM & illustration.

Best luck

Fadi