Computer input devices are anything that you can use to transmit data into a computer. Such as keyboards, mice, gamepads, a microphone, or a touch screen monitor. Each of these have different aspects to consider when purchasing. I will go into detail on each of these below, but first i would like to say that i am generally a fan of logitech products. Their design and quality from what i have experienced are far greater than most of the other brands out there.
Keyboards: When purchasing a keyboard your main focus should be on the overall comfort of the keyboards layout. IF you will be using it mainly for typing, a comfortable design should be your top priority. If you are going to be mainly using it for gaming, the comfort should be there but you will also want to look at the layout of the arrows and surrounding keys. For a media center PC you should look for a keyboard with media controls such as the play/stop/rewind/ff keys as well as a volume knob. Unless it is necessary you should stay away from wireless keyboards. There are too many potential problems that can arise such as interference or battery consumption. If you do require a wireless keyboard you should get a bluetooth keyboard to avoid any interference problems. Also look up reviews on the keyboard you are planning to buy to see if there are any complaints about the battery life.
Mice: Choosing the perfect mouse for you can be difficult, there are hundreds of mice available purchase, each with different designs and features. To start, you should make sure that the grip is comfortable. Choosing an uncomfortable mouse can make it difficult to use, or in the worst case even cause your hand to cramp under heavy usage. A feature that i think every mouse should have is forward and back buttons on the sides of the mouse. This will give you a much nicer web browsing experience since you can go back or forward through pages with your thumb. If you will be using your mouse for gaming, a high DPI usb mouse is the recommended type since the high DPI will make it extremely accurate, and the USB connection will make sure there is no lag. If you decide to go with a wireless mouse, think about paying a little extra for a rechargable mouse as the batteries in mice deplete rather quickly. I have a logitech mouse that actually comes with 2 hot-swappable batteries so you can use 1 while the other is charging so there is no down time.
Gamepads: Right now it seems that the way to go for a gamepad type controller is the XBox360 controller and USB receiver for the PC. Pretty much all games have built in support for this controller so your gameplay experience will be much easier. If you are looking for a flight controller system Saitek is the brand to go with. Their Pro Flight X-65F Combat Control System is a well made complete system with available add-on modules for flight simulator X. For racing games you have a few choices of well made Steering Wheel controllers. Logitech has the G27 Racing Wheel which has a complete wheel, clutch, and stickshift system. If you check it out on logitech.com you will also see the "Playseat" that all of their hardware can be mounted onto. If you are looking for something a little cheaper they also have the Driving force GT for half the price, but it doesn't have the clutch and uses paddle shifters instead. Both of these wheels have USB connections and can be used with the PC as well as the playstation 2 and 3.