(Updated & Correct: 12th June 2013)

In relation to comuter specification suitable for using ArchiCAD, Graphisoft list their own recommended & minimum requirements here but through our own experience in DCS, we feel that it would help our userbase if we clarified, or 'fine-tuned', those requirements for best performance for those using a Windows PC or a Mac.

Windows PC
We have listed our recommendations for a new Windows PC below :

• Processor: Intel Core i5/i7 (Quad-Core)
• RAM: 8GB
• OS: Windows 7 Professional (or Ultimate) / Windows 8 Professional (or Ultimate)
• Graphics: Discrete 'NVIDIA Quadro' or 'AMD/ATI FireGL' graphics card (not an integrated/onboard graphics chip)

To clarify more on the comment regarding 'integrated/onboard' graphics :
Intel manufacture processors such as the Core i5 & i7 range with a graphics card/chip 'built-in' to the processor, frequently known as 'Intel HD Graphics'.
AMD / ATI also manufacture processors in this fashion.
These graphics chips are good enough to use on normal applications (Office, Web, Video playback and even some light gaming) but DCS would suggest they are not sufficient for CAD & BIM.
ArchiCAD will work properly with these graphics chip, but performance will be quite slow.
Whilst this may seem quite technical, it is very important in relation to ArchiCAD performance and would be something very important to check/investigate before purchasing a new PC.

Regarding a different topic, typcially the default hard-disk size in any computer specification will be more than adequate for any user - as they normally start at 500GB - more hard drive space does NOT equate to faster performance.
One consideration (if available in the configuration options) that WILL help in a computer's overall performance (not just with ArchiCAD) is that of a Solid-State Drive (SSD) which is a new type of hard-disk which is much faster than a standard drive, see Wikipedia - but they do cost more than standard drives, so this may be more of a financial decision.

DCS would normally recommend a DELL Precision workstation (simply for the flexibility and customization available), but any brand with the appropriate specification is perfectly acceptable.

Macintosh
(As of March 2013, Apple Ireland have withdrawn the Mac Pro lineup from their website - which would have been the preferred Mac option)
We have listed our recommendations for a new iMac below :

• Processor: Intel Core i5 Quad-Core 2.9GHz
• RAM: 8GB
• OS: OSX 10.8 Mountain Lion
• Graphics: Nvidia GeForce GT650M 512MB

With either a Window PC or a Mac, other extras such as monitor(s), DVD/BluRay, mouse & keyboard and Support Options can be chosen at the user's discretion as they won't affect ArchiCAD performance.