I got a Gigabyte GA-P35-DS3 motherboard.
Some time ago I noticed that Windows only showed 2GB instead of 4GB. I don't know exactly what caused it anyway.
I tried putting in each of the 4 x 1GB RAM modules one by one, and tried every slot one by one, until every stick and slot worked. However, then I tried adding one more at a time, and it kept showing 1GB, until I put in all 4, where it only showed 2 GB instead of 4 (in BIOS and windows 7 64bit).
I tried replacing the BIOS battery since I've read that low battery could cause it. It didn't help though. I also bought 4GB new RAM (yes, it's supported, I checked it), and it's still the same, it only shows 2GB (or 3GB, when I put in 4 of the new and 2 of the old).
I also did the latest BIOS update, and used default BIOS settings, but nothing of that helped. When my PC boots it shows "RAM modules used 2 and 3", when 4 sticks are in - or "0 and 1", when only 2 are in.