I need to do a ram upgrade on a sony vaio laptop (japanese model)
which takes PC100 SDRAM. I read that this model takes up to 512 MB
of ram. Its an old celeron 900 MHz thing.

Laptop ram seems to me too be much more hit and miss for compatibility.

I would have thought thgat PC133 would be ok but what im not sure about
is if it can take single sided or double sided ram or which is more likely to
work. Considering its age in that it takes PC100 what is more likely to fail
double sided or single sided 256 MB ram modules?

The underlined bit has got me concerned as im not sure what they mean.
Does it mean its single or double sided?

I have seen this on ebuyer.

Ebuyer 256MB PC133 144PIN SO-DIMM Notebook Memory

144 Pin SoDimms are commonly found in laptop computers. Each 144-pin SODIMM provides a 64-bit data path, so they are installed singly in 64-bit systems. 144-pin SODIMMs are available in PC133 SDRAM. When upgrading, be sure to match the memory technology that is already in your system .