Can you explain the process of changing graphics cards?

i have the Hp Pavilion a6620f and it has the GMA 3100 graphics card. I want to replace it to be able to run the average retail fps out there but i am not a computer wiz so i dont know anything =[. i posted threads and they said that i had to have a "PCI" slot or something, and a certain amount of watts. can anyone please explain to me why these are significant? also, my budget is 80-100 dollars for a good video card that will run games such as battlefield 2142 or unreal tournament. (must be from best buy)