I've got an Asus P7P55D LE motherboard with 4GB ram, divided in 2x 2GB which are currently in slots #1 and #3. I want to add 8GB (i.e. 2x 4GB) and I know it's usually best have matching pairs of ram.
Now if the existing two 2GB dimms would have been in slot #1 and #2. I would have placed the new 4GB dimms in #3 and #4 without any doubt.
But since they're in slot #1 and #3 (installed this way by the guy who originally built the PC at the shop, and they seem to know what they're doing) I am wondering, how does this pairing work? Should the dimms in slot #1 and #3 be equal, and likewise #2 and #4? Or should #1 and #2 be equal, and likewise #3 and #4 ?
You should never mix different quantities of RAM, the computer cannot use all of the RAM on the 4GB modules if you have 2GB modules added in as well, it will only use half and then you will also suffer performance loss as it will have to map only half of the memory. If you must, put it so that a 2GB and 4GB are one channel and a 2GB and 4GB are another channel.