The more hardware in use at any given time will draw more current (amps) from the CPU. It is unlikely that all your hardware within your computer will be in use at the one time - except probably at switchon - again Startup, advanced will show number of loadups.
The more current a graphics card is, better the results - point to consider is what the computer is required to do - unless current released games and digital photographics is your number one, then to spend a large amount is unnecessary.
A rough guide is to make a note of hardware, cards, motherboard etc installed and then add each item's drawn current - if this total adds up to more than capacity of your installed CPU - you will then have to purchase a higher rated CPU.
