stick in slot 1 and one in slot 3 are intended for dual channel operation. For dual channel the memory sticks must be the same size 'called matched, but don't actually have to be matched, but the same amount of memory]. Are slots 1 and 2 one color and 3 and 4 another color. Apparently Dell numbers them 1,3,2,4 so for dual channel one puts the two sticks next to each other. Most other mobos seem to use the two color system and dual channel goes into 1 and 3 then 2 and 4.
Try putting them into 1 and 2, should then run single channel as 7XX MB.
Hey RProffit
I have just installed the for mentioned motherboard into my computer. I put in 1 256 ddr and one 512. one in slot 1 and one in slot 3. All seems to boot ok when i first turned it on, but then it stops. It was having trouble loading windows, so i selected safe mode. Upon hitting enter, it rebooted. I set by bios to a different boot setting hdd-1 to hdd-0 still didnt do it. still reboots at windows start up. I changed the boot back, disconnected my extra hd, and now i get an error NTLDR is missing and tells me to reboot on my own. I dont know what I did wrong, and am hoping you can help me out. Thank you in advance, David Steinbacher

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