Adding memory to tx pro mainboard

You do not say what memory you are trying to install.

The ASUS P4S800D motherboard has four memory slots that hold DDR PC2700 or DDR PC3200 with a maximum of 1GB per slot, so the installed maximum is 4GB.

DDR memory is generally available in 128MB, 256MB, 512MB and 1GB modules. If you already have memory installed, it must be at least 128MB so you cannot add modules to 'bring memory to 128'.

Either you have a different motherboard and posted in the wrong section, your memory calculations are wrong, or you have used older PC-133,PC-100 or PC-66 memory instead of DDR. The last is unlikely as older SDRAM has two notches and won't physically fit into a DDR slot that only has one notch.

Please re-post with the specific motherboard model just to confirm and a list of the memory modules you are trying to use.