Hi Lec,
There is no one answer, three sources for you to read up on:
1) Read the ArchiWiki on AC12 (click on the ArchCad12 item under "Products"),
http://www.archicadwiki.com, you will find the first two items discuss 2D speed and 3D speed.
2) Passmark,
http://www.passmark.com/index.html, click on the Benchmark tab, you will find speed scores for a variety of CPU's, Video Cards and Hard Drives. See what performance your getting for $ spent.
3) Tom's Hardware,
http://www.tomshardware.com/us/
at Tom's you will find extensive articles testing different memory/CPU/etc speeds vs real world results in applications that vary from simple to loaded renders. Site is mainly for techies looking to improve performance and build their own PC's.
Hope this helps,
Mark