1. Because your motherboard should not support 8GB of memory.
DDR2 SDRAM was introduced in 2003 and replaced by DDR3 SDRAM in 2007. At that time, the prevailing OS was Windows XP, which is a 32-bit OS.
32-bit OS only supports up to 4GB of memory (but only 3GB usable). Only 64-bit OS supports 4GB or more of memory, i.e. Windows Vista.
If your motherboard was made during that time, there is no way that your motherboard can support that many memory.
2. Difference in design.