best Linux for architects

Almost any Linux flavour would allow you to install and run the Linux versions of CAD software. It's more the hardware that you're using, than the version of the Linux operating system that will work best with graphic intensive software.
I use Fedora myself, and, am able to run Blender (3D graphics program) and have occasionally run 3D games successfully. I use the Open Source graphics drivers to accomplish this. With a Nvidia card, it's the nouveau driver.
Some folks think that the Open Source drivers aren't up to the task yet, because they don't include all the capabilities and run slower than the Nvidia proprietary drivers. If you intend on using a CAD application, you might feel the same way, but, I would at least try Linux with the Open Source drivers first, and, see how they perform in your work environment.
Fedora (KDE version) and OpenSuse have the most advanced versions of the Open Source drivers that are tweaked to work with 3D.
If you're not into experimenting, and, if the folks who distribute the CAD software that you want to use recommend using the Proprietary drivers, distros like Linux Mint (KDE version), or Kubuntu have made it easy to install what we call the binary blobs, that are the drivers for various video cards. I know from experience that it is a hassel to get the Proprietary drivers working on Fedora, due to the fact that Fedora is cutting edge, or in other words, it has the most recent versions of the video graphics technology, and, it's hard for the video card manufacturers to keep up with the updates.

OK, now that I've rambled on about graphics and different distros, I took a search and found three distros that actually either come with CAD software, or have been developed to use scientific software, so, you could always look to see if one of these fits your requirements.
http://distrowatch.com/table.php?distribution=scientific
http://distrowatch.com/table.php?distribution=poseidon
http://distrowatch.com/table.php?distribution=cae
Going to these links, and visiting the home pages, you might recognize some of the applications that they come with. Posidon and CAE actually advertise the 2D/3D CAD software in their descriptions. I don't have time to look closer into Draftsight (I'd really like to see how it runs), but, I'd be interested to see what Linux versions and/or distros they would recommend running it on.

Ok, I guess that's a start. If you feel I haven't covered anything, just ask, or, you could also wait a bit, because I'm sure others will be by that have more experience with CAD then I do, and, might have some more specific suggestions.

Sorry, I missed it : www.linux.ie/newusers/alternatives.php