This is what I used. It is by no means a list of what is required
I use a 2010 Mac Book Air running OS X Lion: 1.86GHz core2 duo, Nvidia 320M, 4G ram, 128G SSD.
By today's standards (late 2012) this is pretty ancient, but it's still quite snappy most of the time thanks to 4G ram and an SSD. For this work I hooked it up to a Dell Ultrasharp 20.1" 2007FP and use an Apple bluetooth compact keyboard (sans numpad).
Wacom Intuos4 medium
3D Connexion Space Mouse Pro
I enjoy 3D work, hence the space mouse pro. If you download the latest (currently Beta) drivers for it, it becomes much more customizable and a great help with other applications.
ArtRage Studio Pro
I'm a huge fan of open source software and wanted to use it with this project, but just couldn't find an application that I found easy to use AND readily compatible with OS X (ie. officially supported for mac). Then I stumbled across ArtRage. I am not an artist, so I can't give qualified advice, but I love it and at (the current price of) $60 for the pro version, I think it's affordable. I don't regret this purchase at all.
That said, the rare times that my mac book air does slow down, it's using ArtRage. Drawing/painting is fine, it's more saving or layer manipulation where things pause. It's a little bit annoying, in this day and age you don't expect file saving to take 20 seconds, but it is what it is. I'm working at a resolution of about 2100x2500.
Poseable for Mac
Full disclosure: Poseable is my app for Mac and iOS ($1.99). I made it to study anatomy and help me learn art. It's not an all encompassing 3D software package. It's a simple app meant to help with posing/blocking out scenes, aiding with composition, perspective and human anatomy. I also have a free Poseable Lite out, but that app doesn't support scene creation with multiple figures and props, which is kind of needed for this work.
There are a number of posing applications around which can do a comparable or better job. Some are free, some are not.
Setup (spend some time on this!):
Drawing a picture for fun every once in a while is one thing, illustrating 18 pages for a book you'd like to sell is another. Speeding up your workflow can pay back hefty dividends down the road. Depending on what you're illustrating, each picture could take 5 hours or more to paint. If you can speed up your work by even 10%, that's an appreciable amount of time saved. Also worth noting, the less tedious it is to create a painting the more fun the entire job will be.
For this kind of work my setup is a bit unique (tablets are common, space mice, not so much), I found it took a bit of time to figure out what quick keys were best assigned where, so perhaps someone will find this useful.
I specifically bought the smaller wireless Apple keyboard so that my desk could accommodate both a tablet and keyboard comfortably. Adding the space mouse hasn't caused any (desk real estate) issues. When I'm illustrating I usually push my keyboard up and bring the space mouse and tablet together.
Please see the attached images for my quick key layout.