The key is to have the same speed in each bank. If slot 2 and 3 are one bank, then you might be OK.

check your manual or check the motherboard online and see which slots make up which banks. Sometimes they are color coded but you cant rely on that.

Edit: If it beeps when you turn it on, count them and then look up the error codes from the short/long beeps.

It will only run at the slowest speed but unless you are playing a memory intensive game or using Photoshop to its max, you shouldn't notice.

Usually, slots 1 & 3, and 2 & 4, are paired. If both 2 and 4 were 8GB or both 4GB, some really memory-intensive applications might run faster. If you're not doing high-end image processing or scientific computation, you'll probably never notice. Routine Web browsing etc. should not be affected to any degree you could notice.

ETA -- Oops, didn't even notice the clock speeds. Try swapping DIMMs 3 & 4. Then you've got 2400's paired w/each other, and 2666's paired w/each other. Right now, NONE of your DIMMs match in both size and speed. If you at least pair them by speed they will run at their max rated speed; as it is the 2666's will probably be "clocked down" to run at 2400.

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