I have a p4 2.93gh prossesor 1gb ram computer.so which graphic card I should buy..?

A Pentium IV while a great chip is not what it takes to play GTAV. one needs at minimum a e6600. so you would need a minimum new motherboard, RAM and CPU. Also you need at least 3 GB of RAM and have to have windows vista or newer. this will all be for bare minimum. also you would want a power supply that can support a card like the GT 720 or better, so at least 300 watts. I think you may want to build a new computer.