I will start by saying that I will avoid the whole Mac vs PC debacle. Suffice to say that with Shake being dead, the case for owning a Mac purely for compositing has been dealt a blow. Of course Nuke runs on Mac (quite nicely apparently) as does 3D software such as Autodesk Maya (sub-par compared to PC and *nix) and Houdini (stable, from all reports).
Here’s what I consider the minimum gear to work quickly in CG:
Two Monitors
The monitor of choice
This should be a no-brainer, but if you haven’t experienced the joys of a dual monitor setup, you are really missing out. Thankfully these days even lowly video cards have dual DVI-outs, so it is as simple as inserting the card and connecting up your monitors. Some software makes good use of the dual monitors with the second being used as a reference monitor, such as The Foundry’s Nuke, while others pretty much just use the second monitor as a storage space for all your floating palettes (Adobe Photoshop).
Other than that, dual setups are perfect for training as you can have the video open on one monitor while following along on the other. Watching video training on a single monitor is about as fun as having starving penguins kick you to death and then eat you.
What monitor? Frankly, for the price you can’t go past the Dell 24″ 2408WFP. Sure it might be a little more expensive than others, but it has excellent colour reproduction thanks to it’s 8 bit panel. Most of the cheaper monitors use the cheaper 6bit panels as far as I know. Keep an eye out for weekly sales and scour the internet for extra 20% off codes that are sometimes emailed out. Combine them and get a great monitor for peanuts. Of course, if you are just doing non-production level stuff at home, there are a bunch of good 24″ models these days and prices have plummeted. SAMSUNG, ASUS and BENQ put out a couple of good ones. ACER seems to be a bit lower quality to my eye.
Tablet
In the words of They Might Be Giants, S-E-X-X-Y
It might be hard to believe, because I am so awesome, but for some bizarre reason I used a mouse with Shake for the first year of production. Why? Buggered if I know. I tried using a tablet a couple of times but I found it quite unwieldy. When I changed studios and was essentially forced to use one, it took me about 2 solid weeks for it to feel totally natural. Remember that I had been using a mouse for maybe 18-19 years, so cut me some slack! I have since started using the tablet for my 3D work and again, while unwieldy at first, you soon get the hang of it.
Check out the new Wcom Intuos 4 for some freaking sexxalicious tablet goodness. Do you need the latest thing? Not really. Keep an eye out for run out stock of the excellent Intuous 3’s. At this juncture I would only really recommend Wacom. I know some people, especially students, are tempted by cheaper products, such as Genius, but really, you get what you pay for and you can claim it on tax. Tablets and monitors are like shoes . . . you use them everyday and they are critical, so don’t sell yourself short but buying cheap gear.
Video Card
Any decent gaming card. It has been well established that Quadro’s offer negligible performance benefits for the average user. Get your studio to buy them if you desparately want one, and even then I think they would baulk at shelling out the kind of cash nVidia ask for those things. Some people may swear by ATI, but nVidia just seem to be more stable across all your needs.
Chair
My bottom wants to meet this chair
Chairs? What the heck are you talking about Matt? Who gives a sod about chairs? I admit that this is a luxury, but only to a certain extent. Like a monitor, a chair is something you will be using pretty much every minute of your working day. You are sitting on your arse all day, so why by uncomfortable? The Herman Miller Aeron is the King of Chairs for the arduous bum-sitting marathons that are found in CG. Expensive as all get out, but your cheeks deserve to be coddled.
And now for the pure insanity of Stefan Didak’s setup. ”Look on my works, ye Mighty, and despair!”
Stefan Didak presents "Gentlemen Start Your Monitors"
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Anyway, I am going to list the websites and podcasts that I regularly check out. I must confess that while I used to listen to music pretty much 24/7, these days I am more likely to listen to podcasts and audiobooks while I work. Stuffed if I know how some guys can listen to Finlandish death metal with the volume at 11 while working, but different strokes for different strokes. 
Of course you have couriers and a myriad of other people, however the drives etc are usually pretty secure.