If your original memory is 5 12M, it is enough to insert 1 2G ddr2 800 (old 100 yuan) memory, and the old memory of 5 12M is not needed, which is prone to compatibility problems. It turned out to be 1G 667 memory. Buy 1G memory.
The machine is too old to use 4G memory, and XP system can only recognize 3.25 G.
Please clarify the concept of ddr2\ddr3. The two are completely different memories. What memory does the motherboard slot support? Then, this slot can only be of this type, either DDR2 or DDR3, and the two cannot be mixed. The motherboard slot has a foolproof port, and the memory you want to insert cannot be inserted.
Backward compatibility refers to the same type, that is, ddr2 800 memory can be plugged into a socket that supports ddr2 667. Similarly, ddr3 is also compatible on the basis of DDR3, not DDR3 compatible with ddr2 as you said.