A used one with the minimum requirements on the PC game disc for Call of Duty 4 Modern Warfare
yes
Yes, most definitely
if you have a computer or playstation3 then yes, xbox360 costs money
if you want it online,go with amazon.com, if you want to buy it at a store, definitely Gamestop
yes i i have played call of duty on a computer
Yes, you can level up online just like for the xbox 360, ps3 and computer. I know this for call of duty 4 and 5 for the wii but have never played call of duty 3 or prequals so don't know about them.
if you have a computer or playstation3 then yes, xbox360 costs money
Yes, there are several Call of Duty games released for the PC. Yes if you have purchased the computer game disc or download for that Call of Duty game.
The best one would have to be Call of Duty 3 on PlayStation 2. Yes you can go online on it.
All of the newer call of duty games can be played online. I've been playing since Call of Duty 4, and they are making Call of Duty 8 right now, and all have been online enabled.
if you have the computer game or PC version for Call of Duty World at War
no