In practise, ZG5 can only accept 1,5 gigabytes. This is due to following:
* the chipset limitation is 2 GB
* there is 512 MB (0,5 GB) RAM soldered onboard
* there is only one memory slot
So, if you add a memory stick of 256 MB (0,25 GB), 512 MB (0,5GB) or 1 GB you'll be fine as you don't exceed the chipset limitation. 1,5 GB memory sticks don't exist. If you add a stick of 2 GB (or more), you'll exceed the chipset limitation and your ZG5 won't boot.
There are rumors of physically removing the original onboard RAM and then adding a 2 GB stick but I don't personally know anyone having done that. Anyway, I wouldn't recommend it, it would require some skills with soldering iron and such. I also have a vague memory of someone talking about newer ZG5 units having 0 or 1 GB onboard memory, so giving a possibility to reach the 2 GB limit, but I only have met older ZG5's with 512 MB onboard.