Buy Wacom Tabs

For a starter level I would recommend this:

It gives a decent working space, can be easily carried and good for getting used to using the technology.

However, if you are sure what you are going to be doing with the tab and will be spending a considerable amount of time on it. This is definitely a one time investment. I would recommend:

The Intuos Professional Pen & Touch is a versatile tablet chosen by creative professionals. Combining Wacom’s finest pen capabilities and multi-touch gestures, this medium-sized pen tablet gives you ample room to design, sketch, paint, retouch, and create with ease. Connecting to your PC or Mac through a single USB connection, Intuos Professional Pen & Touch delivers 2048 levels of pen pressure sensitivity and tilt recognition to let you create with the accuracy and precision of traditional brushes, pens and markers. Combine this with multi-touch gestures that allow you to position and navigate your work intuitively. Get the most from creative software with your favorite shortcuts. The Intuos Pro detects the software you’re using and offers optimal short cuts and selections. Quickly access all your shortcuts through eight customizable ExpressKeys and the Touch Ring. Stay focused with the Express View heads-up display and Radial Menu. Discover intuitive navigation at the touch of your fingers: pan, zoom, rotate and navigate naturally with gestures, or create your own multi-touch gestures. Cut down on cord clutter and enjoy the freedom of creating wirelessly, with the included wireless kit. Intuos Pro is designed ergonomically for both right and left hand use, so you’ll be comfortable all day.

Additionally, to the above, Wacom has launched a new technology for serious illustrators who want to get a natural feel and yet use the advantages of digital art. Basically, they use a screen on which you can directly sketch. This may take some time to get used to and compared to others it is heavily priced. Maybe you would want to buy this later after you have got used to the softwares and the concept of digital art.