Have you resolved your issue?
So, you have been replacing components, moving components in and out? Each time do you go into your bios to make sure that the computer has recognized the component(s). Check to see if you have a jbat1 or a reset. If jumpered usually with a different color, like white of read, with one exposed prong and two covered. covering the other two prongs for an instant resets your cmos settings to factory default. If you decide to replace your mobo you do not have to get the same exact board, just make sure that all your components are compatible.
When you turn your computer off by the power switch in the back of the power supply instead of using the power switch on the front of the computer, you are essentially interrupting the power source not turning it off. Its the same as a power outage. You loose unsaved data that way. You may want to check your RETURN TO STATE AFTER POWER LOSS.
When you only installed 2 out of 4 memory sticks, did you install them starting with BANK 0. Also did you remove those same 2 sticks and replace them with the other two sticks, each time checking bios/cmos to make sure they are recognized. Sometimes it takes more than one boot to be recognized. However, they must be recognized before you proceed. If 2 sticks are required to make a bank then you must insert 2 sticks at a time. I hope you are practicing safe repairs/diagnostics. Wearing a ground strap, turning the computer completely off (disconnecting power cord), discharging yourself before handling sticks, etc.
When you switched the ram did the second pair work in those slots?. Then you plugged all four in and it didn't work, correct? Did you try putting the ram in the first two slots (farthest from the edge)? Each time checking to make sure the computer recognized the memory?
Do you have a cmos battery?
