I have an old Dell 4700 using DDR2 SDRAM. It has two 256K memory sticks in it currently. I found some 512K sticks that I was going to install but after removing the original sticks, I found that the 512's wouldn't fit. So I put the two 256 chips back in but now it will only recognize one of them. I see no damage on either and I'm not getting any errors on boot. I switched them back and forth in the four available slots and it still only recognizes one.