You need Double-sided Low-Density (more than 4 chips per side) for a total of 256MB.

So if you get a compatible 256MB SoDIMM, it needs to be PC-100 SoDIMM with 8 chips per side. Double-Sided 4 chips per side is High-Density.

I have experienced this. I have 4 of these DIMMs but they are all used. I found that the only type of 8 chips total for 256MB I have come across are all high density, so you would only get 128MB. My Pismo will use both high density and low density RAM, but the lombard has the same issue as the PDQ. Required double-sided low-density.

Easy way to tell compatibility if the physical chips are from Hynix/Kingston/major brands, has 16 chips totaled from both sides.

You will notice that 512MB High-Density takes 16 chips on each side and basically puts 256MB Per-side.

Hit ebay and look for 256MB Chips that are PC-100 (!Avoid 133, most 256MB even if both sides are high density if the 256MB is 133Mhz!), use both sides, and have 16 chips total. Easy way. I will look for some chips on ebay and post links

EDIT: This will work for you just fine.