First, I wouldn't buy a new Mac Pro today unless I had no other options... the Mac Pro has not been updated by Apple for years - and is finally going to be updated in 2013.
6 GB memory is kind of an odd number ... I'd take no less than 8 and ideally 16 (or more). Third party memory (macsales.com) is as good as and way cheaper than Apple memory.
A 2012 iMac can deliver the same performance as the configuration you list. If you need additional drive bays, it supports Thunderbolt expansion. A core i7 with 4 cores gives you 8 virtual cores for multithreading.
For Artlantis, the more processors the better. You won't see as much benefit with Archicad. Rather than configure every workstation to be a max-speed Artlantis machine, it might be more cost effective to have one 'super' Artlantis machine - and the others a 27" 2012 core i7 iMac with fusion drive - which will be great for most uses.
For the super Artlantis machine, I would look at a Mac Pro with as many cores as possible, and at least 16 GB RAM ... but would live with the iMacs until Apple releases the new Mac Pro next year. You might even decide you don't need a Mac Pro...
K
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB