I tried it alone and the machine booted up fine, but when together, in either order, the system won't boot up. I've tried installing them into DDR1 and DDR2, DDR1 and DDR3, but the results are the same (The 4 184-Pin slots are labeled DDR1/DDR2 and DDR3/DDR4). Either A) A long beep every couple seconds (Supposedly signifies a Refesh Failure); or B) The system boots up, runs a memory test, says the memory checks out and then doesn't boot up the HD. When in DDR1 and DDR3 the system doesn't even boot up, just lets out one long beep one after the other. The beeps are supposed to mean Refresh Failure, Parity error, or they could mean 64k memory error.