Just updated my mobo, CPU & memory; REAPER locks up my system

^^^Actually, I'd say make sure that all of your drivers are up to date, including chipset, video card, BIOS etc... Any system can be a buggy P.O.S. if the system drivers aren't up to date.

First thing I would check are the drivers - audio interface, processor & chipset, etc. However, my gut feeling is that your memory timings in the system BIOS are wrong. The motherboard auto-detect of CAS/TRAS settings on RAM seem to be very commonly mis-detected. Lockups are the primary symptom when your BIOS memory timings are wrong. Have you had any lockups outside of Reaper? Or is this a pure DAW that you don't put through its paces doing anything outside of Reaper?

do your PCI cards work with other software on your current machine? I upgraded my old MOTU 2408mkII (PCI) based machine, only to find out they'd changed the PCI standards in the meantime from 3V to 5.5 or the other way around and my card didn't work properly. probably not the issue here, but just a thought.