You are fine to stay with 1600mhz, the 2133 speed difference isnt that great , which is also dependant on the timings.
Just give your 3570K a good overclock and dont waste your money replacing your ram for a small increase in performance.
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.
Ideally 1.50v is best with a VCCSA (VICCO) of 1.1v-1.2v. CAS 9 with 8GB/stick is faster but requires more voltage to remain stable. Using any of those kits set use the e.g. ASUS AI Overclock Tuner -> XMP then save and everything will be set optimally.
Any of those kits are good and safe long term, 1.50v is safest.