SY-P4VGM beep on power-on

Hi again - regarding the SY-P4VGM mobo I asked question on USB header.

Managed to install XP fine, but upon reboot before screen came back on,
motherboard gave one long beep (about 4 seconds) with a short gap and
then repeating continuously. It will not reboot.

Is the RAM stuffed (256Mb DDR in 1 stick)? Do I have to install DDR RAM
in pairs?

Any advice appreciated.

Thanks,
RoyJ