There is no, as such, a recommended input device to use with your media center. You want to buy something that you like, something that fits your bill. If you look closely on the back of your system, you will see several USB, and two PS/2 ports. Most input devices adhere to USB specification, and so the choice of keyboards, mice, and remote controllers is virtually unlimited.  But since we're talking about media centers, we can cut down our choice to only wireless devices.

We normally go for Microsoft and Logitech wireless products. Both brands are readily available in stores. For remote controllers, Logitech offers everything (search for Logitech Harmony), whilst Microsoft offers the better keyboards/mice for media center applications (for ex: Microsoft MCE 7000 combo).

You don't have to buy the more expensive products, most input devices behave roughly the same.