Look, if all four sockets are working with the i5 processor, then we know that the two memory channels implemented on the board, as well as the four DIMMs (two per channel) inserted into these memory channels, are all working properly. If we then see only one of these channels actually working when a different processor is inserted, then it is that processor that has the problem.
Repeating, two is the correct number of memory channels. The board and processor (are supposed to) support two memory channels and two DIMMs per memory channel.
How dare I? I am not saying anything that isn't completely true. You need to be setting your expectations accordingly. This is an ancient, 12-year-old system. The MTBF for every component on the board - including even the solder - is long past. The same applies to almost every other component in the system - drives, power supply, graphics card, etc. I suggest that you be very careful what you spend on this system. I consider it throwing away good money for bad. I would be putting up with it, as it is, until you have the finds saved up to replace the whole system with something more, um, contemporary.
...S
