I upgraded the Value Select RAM (DDR2-800) in my P5N-E SLI motherboard by adding more DDR2-800, but it isn't working.
If your system currently contains 2048MB Corsair Value Select DDR2 PC6400 DDR2-800, and you add 2 more sticks of RAM to the system, it will not work properly. The Value Select model RAM when used in this motherboard is not compatible for use in 4Gb configurations. This motherboard's RAM specifications are more strict at the 800 speed, requiring you to purchase 4 new sticks.
Refer to column C of the Qualified Vendors List (QVL) from Asus for this motherboard for modules that have been tested and should work in a 4Gb configuration. You may purchase RAM that is not on the QVL, but your level of success may vary. Be advised that we recommend RAM that is CL5 (5-5-5-15 2T) and 1.9 volts, like the Corsair XMS2 DDR2 PC6400 DDR2-800 (CL5 1.9v).
Note: When upgrading RAM from 2GB to 4GB, you may not get what you expect. More details here.