A Wacom is the only graphics tablet brand I'd ever recommend. If you can afford it, get an Intuos3; if you can't, the Graphire is still wonderful. I've been using one model or another for over 10 years, and am currently using the first Graphire model (ET), which I've been using for the past 8 years or so.
vohdoun is right that choosing the size that's right for you is important. wacom.com has a wizard that'll help you decide which size would be right for you. For most people, the 6x8/6x11 is sort of the "sweet spot," but I know people who have found it uncomfortable and prefer the smaller size. It really depends on how much space you have, how you're planning to use it (on your desk, at a short distance from the computer and leaning on your knee, whatever), and your wrist/arm movements. After you've used the wizard to help you pick a model, definitely try to check it out locally if you can. Spending a few minutes with one certainly won't help you make a definitive decision, but it's a start.
One thing. For most people, it's a little while before they consider a graphics tablet indispensable. In the beginning, people frequently find it uncomfortable to use, and also find it disorienting. Remember that unlike with paper and pencil or paintbrush and canvas, you're looking at one spot (the screen) and drawing elsewhere (the tablet). It definitely takes a little getting used to. Allow yourself the time, don't get discouraged or frustrated. If you use a mouse right now, or a touchpad, use the tablet frequently for 2-3 days, and then try to use it full-time for another 3-4 days. At the end of a week of solid use, you should be able to determine a) whether a tablet is right for you and b) whether you've bought the right size for yourself.
It really does make a world of difference, especially in an application like ArtRage. If you're like most of us who use a tablet, you'll soon wonder how you got on without one. Have fun, and good luck. =)