You can add a 512 MB stick if you can get it. It does not have to be the same speed (e.g. 33 MHz),
but its recommended that it is. Different sizes will work, but again, its recommended that both be the same.
That said, I have 512 MB (2 x 256 MB) of SODIMM RAM.
ProMOS Technologies DDR333 (PC2700) - CL 2.5.
Came out of a HP-Compaq laptop I once had, so I know they work.
You can have it for $400. No warranty though.
Interested?