In fact i there was just a thread on here where everyone was telling the OP that his new ram was causing his hang up issues, and that he should use only one type of ram.
Was they right at the end? Did OP reported back that RAM was a culprit? I remember that thread, BTW. Mixing RAM manufactures rarily causes issues as long as their specs do match. I can tell you this from my personal experience (and not only me).
Used RAM, I agree – you never know how long it will last. But on eBay you'll find brand new sticks with warranty and return possibility (check this one I linked in my post above, for instance).
I'm out, to not derail this thread more than I already did