You'll probably want an 8" or 10" tablet (I have a 7" and it's a little small for drawing duty), it will have to have a resistive touch screen to use a stylus, and you'll probably want to use the MaplePaint app (it's the best drawing app I've found for Android so far). This is fine for quick sketches, cartoons, etc., but not really for professional-level digital art.
If you want anything more complex than that, you'll probably be looking at a Win7 tablet or convertible netbook/laptop that can run professional drawing programs.
The only one I have any experience with is my own - a 7" Haipad M701-R. They also make a 10.1" model. A lot of what you get will depend on how much you're willing to spend. Also, you'd probably want to dive in on an Android 2.2 "Froyo" device if you can - I know if I were buying right now that would be a minimum requirement. Additionally, if you want a device with any kind of support you're probably NOT looking at one of the cheap China tablets - you'd be looking for a "name brand" device.