Okay here's the basics on pen tablets.
I've used quite a few and none other is actually worth your money than an Intuos and Wacom tablet, both made by the same company and both relatively expensive.
Though 100$ for a good pen tablet is always worth the expense.
Wacom aka Graphire4 or lower would be your basic model ranging around 99$-112$ depending where you get it.
Since they are usb based equipment it works great for macs and pc alike and Vista ready if you're a new Windows OS user.
If you're just starting out or not to picky of having a large work surface than a 4x5 or 5x6 wacom would be the perfect thing for you.
Difference between the Intuos and Wacom is the sensitivity of the pen,
It's also a huge difference in size and price and quality, like $200 difference.
They range from many different sizes.
Intuos is a tablet more experienced CGers who depends on the sensitivity to do their work. So if you do soft shading style art or use programs like Painter Corel this would be something you may want to invest in the future.
For me I user a Wacom graphire4 something much smaller and basic.
despite it's smaller work area a 4x5, I never seem to have a problem since the work area sizes itself with whatever program you're working.
Remember along with the tablet you also get programs so chalk it up that most of the price also includes 1-2 good programs to use.
Art is also about technique, how you execute and use your tools can mean a big improvement in the way you draw. So ask lots of questions!
Oh if you want any tips feel free to ask me, I'm a big squishy pool of knowledge, at least when it comes to photoshop. =P