for doom3 and most fps games my advice is to stick with with the keyboard and mouse, if your not used to it you soon will be as its easy to pick up after some practise.
game pads are good for some games like grand theft auto click here etc that are converted from console versions but this plays well even with mouse and keyboard, depends on the game and how its converted to pc.
a joystick in my opinion is essential though for a flight sim game and i would spend money on that if you use them. other than that keyboard and mouse rules.
logitech, saitek or microsoft sidwinder products are usually good quality click here click here