Dell Precision 390 16gb DDR2 ECC memory upgrade

Hello there, i have 2 dell precision 390's that i would like to upgrade their RAM. I recently upgraded the CPU's to a Intel Core 2 Quad Q6600 2.40GHz and i was wondering if i could do the same with my RAM. Now to my belief i think the chipset in the dell optiplex 755 is the same in the 390 and it could handle 16gb ddr2 Non-ECC. I found a set of 4x4gb DDR2 400MHz on Amazon.ca and i was wondering if the 390 is able to step the speed down to 400MHz or whatever. I also found a set of 2x8gb DDR2 667MHz ECC. Now my last option was this, a set of 2x4gb DDR2 800MHz Non-ECC (i would buy two of these to make 16gb). Which one of these configurations will work on the Dell precision 390?

4x4gb ddr2 400MHz ECC:


2x8gb ddr2 667MHz ECC:


2x4gb ddr2 800MHz Non-ECC:


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