iMac is perfect for an artist. Though as mentioned here, reflection on the screen....IF the next update of the iMac has something which are more anti reflective I would consider waiting a couple of months for that and you probably get more for your money then too.
I´ve worked professionally as a digital artist for 20 years. I use Wacom and it´s the best tablet you can get, without doubt (I have tested others).
I´ve been using the Intuos model, currently on Intuos 4. This is the professional line from Wacom, and its a great product. I dont know how much money you are willing to invest, but if 349$ is ok then the Medium Intuos 5 Touch is perfect:
http://www.wacom.com/en/Products/Intuos/Medium.aspx
But if you want to do it as a simple hobby, the low end wacom tablets are great product too. The current low cost model is actually good products, despite the low cost - in fact you get more from the bamboo serie today than what Intuos had to offer 5 years ago. So hobby work, no need to think you can achieve less on this or that drawing a nice drawing is harder on a Bamboo. I would go for the 199$ larger bamboo:
http://www.wacom.com/en/Products/Bamboo/BambooTablets/BambooCreate.aspx.
But even the small models are fine if you just want to play a bit. They dont have multitouch and eraser baktip, but I´ve never used those anyway...and the buttons on the tablet is also something I never used. so you can easily create good stuff with a cheap 79$ bamboo too...and you still get pressuresensitivity:
http://www.wacom.com/en/Products/Bamboo/BambooTablets/BambooSplash.aspx
Or of course, if money doesn´t matter and you wanna go pro at once, the Cintiq´s are a completely different ballpark! I´ve just ordered a Cintiq 22HD, and I can´t wait. But it´s a different pricetag too though, and they are huge...so no point unless you are a professional.
http://www.wacom.com/en/Products/Cintiq/Cintiq24touch.aspx