Chips are M5M4V16400BTP-7 which ID as 4mx4 apparently.
Doing the maths results in 256mb and the card is labeled "DT256" but of course according to available info neither the 3400/5300 or the PB G3/Kanga can use a 256 module. So that can't be right. So I'm doing the math wrong.
Yes, your math is correct (as you discovered), but you neglected to convert megabits (Mb) to megabytes (MB, or MiB), so, yeah. Easy mistake to make I think.