First of all, get an apple. Find a qualified Apple Dealer and they'll easily be able to help you. Apple's are far superior for any type of multimedia design, from graphics, to video, to sound...
Generally with Apples, they come in models with a few customizable options. With graphical work and CG rendering, the main items to look at will be the CPU, the amount of RAM, the graphics card and the monitor. Most people forget about the monitor but it's a vital piece of graphical creations. You need to be able to get the highest possible resolution to fine-tune the small details that make the images really stand out.
For a good starting point, you can check out the Mac Pro at
Apple - Mac Pro
8-core CPU will be beefy enough to handle just about any CG work you can throw at it and the graphic card options should all be more than sufficient.