The difference between the totally stable DDR3-1600 and the once in a while error(s) or worst BSOD from faster frequency RAM makes little sense to 'me.' The gaming difference might be at best 1FPS gain or a few minutes (if you don't BSOD/error-out) on a large render. AutoCAD sucks-up RAM depending upon the project size and with 32GB you have the option for a persistent RAM Drive - http://www.superspeed.com/desktop/ramdisk.php
which will speed-up everything i.e. greater reduction in rendering time...
Instead of 'Faster ... More' - I would recommend at minimum 2x8GB (just in case) set or 4x8GB set of DDR3-1600 CAS 9~11. RAM is dirt cheap these kits only ~year ago were $1500~$2000 and now at $220~$260 are a steal.
(low profile) Mushkin Enhanced Blackline 32GB (4 x 8GB) DDR3 1600 (10-10-10-27 @ 1.50v) Model 994055 - http://www.newegg.com/Product/Product.aspx?Item=N82E16820226317
(low profile) Mushkin Enhanced Blackline 32GB (4 x 8GB) DDR3 1600 (9-9-9-24 @ 1.60v) Model 994050 - http://www.newegg.com/Product/Product.aspx?Item=N82E16820226315
(better IC's but tall for some HSF) CORSAIR DOMINATOR 32GB (4 x 8GB) DDR3 1600 (10-10-10-27 @ 1.50v) Model CMP32GX3M4X1600C10 - http://www.newegg.com/Product/Product.aspx?Item=N82E16820233229
/edit I guess I missed the bell.