DO NOT use any other brand besides Wacom. My boss tried to cheap out on this area and we ended up being guinea pigs for every other option available. Poor OS X support (if any) unreliable pressure sensitivity, buggy software, etc. etc. etc. We ended up with Wacom in the end. I cannot stress this enough - DO NOT buy any other brand.
That said, I have a trio of Intuos 3 tablets. 2 - 9x12 I use everyday at home and work and a 4x5 I take on the road with me. Which model you get depends on what you are using it for. Are you just starting out and just touching up photos/learning Photoshop, drawing programs, etc? If so you could probably get by with the Wacom Graphire. If you're a bit more advanced , want greater pressure sensitivity, etc. go with the Intuos 3 line. A 4x5 will work well with any size screen. If you can afford it go larger if portability and desk space isn't an issue. Any tablet will take a bit of getting used to but once you get the hang of it you'll love it.