Which features must be available in a PC or laptop for the graphic design?

I advise you using (mac) if you want laptop .... or workstation PC if you want pc ...i suggest workstation HP Z600 I used it and it is cheaper than mac ....With  my best wishes .... good luck

High RAM speed, Fast processor and more then @GB graphic card, if you are working on adobe photoshop or permier etc you must have more then 50GB of free space in OS or as much possible, bcz of caches.

You must have as much ram as possible and a good graphic card. Without these 2 dont think about getting maximum benefit from softwares.

Well it actually depends on what programs you are planning to use and your budget. If you do have some cash set aside a MAC can do wonders but you need to realize that upgrade options are limited on the MAC whilst the PC has multiple upgrade paths that you can go on. If you have the space for a Desktop it is the best way to go. Laptops with higher specs are still a bit expensive. From what i can see now you can get a high end Desktop for much cheaper.

For programs like Photoshop, Illustrator etc. the more RAM you have the better. These are memory hogs as they use the RAM as temporary storage while you are working on editing and such. Have a look at the list below which i think are some options that are required for good performance while designing. I have based these options on the desktops that i am currently looking at now.

1. RAM - Minimum of 16GB (these are quite cheap now)

2. SSD - Super Fast Storage Drives (These are slowly replacing our mechanical hard drives but are still on the expensive side)

3. A Fast CPU - Intel Core i5 / i7

4. GPU - Nivida or ATI (A high spec model is not essential unless you are doing a lot of 3D work and playing games)

This is just my 2 cents but should provide a good starting point for you. Hope it helps.

illustrator Photoshop and 3d rendering  

a system with good graphics card , ram not less than 4gb  and good processor