That board is not able to handle more than 2 GB total memory:
Single-channel memory architecture
2 x 240-pin DIMM sockets support unbuffered non-ECC 400/533 MHZ DDR2 memory modules
Maximum HP/Compaq approved memory is 2 GB
If you want to utilize the 800 MHz FSB..you will need to upgrade the memory to DDR2 PC2 6400 i.e. DDR2 800..