sounds like a great system.
i have a retired Tyan (workstation) motherboard that i bought to go with a SCSI controller that i bought to go with a SCSI-160 drive about 3 years ago. i love it, but - it's expensive. eats a lot of electricity too.
i have a little CAD experience. (starting in 1988, currently training on 3D studio max, background in engineering CAD).
the ECC RAM correlates to a dual processor system (e.g. xeon or opteron 940). it's understood that you want to minimize crashes & freezes - i think you can do this with "normal" RAM, DDR in this case, for example the OCZ DDR Platinum.
http://www.newegg.com/Product/Product.asp?Item=N82E16820227210
i just bought a dual-CPU opteron system, so far just using it to back up the Max system. it slurps the files off the Asus P5GD that runs Max. that is, copies files in 10 GB chunks without blinking, drag and drop 10 GB as easy as if it were 100 MB. normally backing up is not fun, but when it's got that neck-snapping whip-cracking acceleration, then it's kind of fun.
i think the Asus motherboards that have the 8-phase power, the Asus A8N32 for example, are a good candidate for your system.
i used a mATX motherboard from MSI for my own dual-core Opteron system, but you're talking full-size ATX or maybe E-ATX, sounds like.
Asus P5WD2-WS Premium - one of the things that makes it a great MB, the 8 phase power. not just a marketing gimmick, notably more stable voltages under heavy load. same for the Asus P5N32.
those 2 intel systems (supposedly) share the power-processing part of their designs with the Asus A8N32.
gaming is not a bad background since it is CPU and GPU intensive. the CAD system is not quite so GPU intensive. i think the expensive CAD cards are un-necessary, a way to make money off nervous engineering managers.
i use an Asus X850XT on my Max system, very rarely hiccups. under $200 at last glance, $400 if you buy it from Asus.