Just about any digital camera can be used to take effective pen photos. If you already have a digital camera, it will almost certainly do for you - at least for a while. I'd recommend pushing its limits before springing for a new camera just for pens.
If you don't have a digital camera already, a decent general-purpose one won't cost too much, it will work fine for pens, and will also be great for other pictures. Just make sure it offers manual white balance, shutter speed, and aperture (f-stop); most point-and-shoot cameras have these features.
If you want to go a step up (several hundred dollars), you can go by on-line reviews to choose one. In addition to the above features, it's probably most important to focus on optics quality.
Beyond the camera, you'll want a gray card, a white card, a tripod, a light tent, and some bright lights (several with the same "temperature" -- that is, the same hue to the light). If you already have a camera with manual settings, these will get you far more improvement than a new camera.