Check out VectorWorks ARCHITECT from Nemetschek. It has a professional price to go with professional features. They have education prices too. An architecture student I know recommended it when I asked him about your question.
I use(d) Vectorworks in OS X on a dual 450, and it's perfectly spiffy. A dual 867 would be more than adequate, and I'm sure an iMac would probably do fine as well, but I feel that the program will benfit from avhing two processors. However what will make the most difference with a CAD program is RAM. Max out the RAM. I know when I use Cadkey 99, RAM was the deciding factor, I used it on a Pentium II running at 600 MHz, and it was dog slow...until I upped the memory. Also, being the program remembers a TON moves/unmoves, the program itself practically runs within the RAM. So in case I wasn't clear, GET MORE RAM!