I would highly recommend NOT mixing and matching sticks of ram, this has never ended well for me, particularly since most everything is dual-channel configs these days and really need to be paired exactly per bank. If you do/must, match them up as close as possible in terms of speed, cas/ras latency, etc, or only use the exact same ram down to the model number.
I've long ago stopped mixing and matching sticks of ram for weird pc crashes, I always put them in as a set now. Matched sets by batch are best, or if needing a bunch of sticks for a server, buying a batch of same sticks used of ebay out of the same server has been good to me too, just make sure to run a memtest on them first to make sure they're good if used.
Likewise test if using off brand random sticks, they're probably cheap for a reason ie. 2nd rate parts. These days I only use samsung memory.
-mb