It sounded like a notebook but just making sure. You forgot the model number however.

That helps when going to look over the specs and seeing what it supports. Another 512mb of memory wouldn't be a problem there. But you first have to know what type is used. The closest match without a model number is seen at
http://stores.tomshardware.com/search_techspecs_full.php/masterid=4577241
One place where you can select the memory by entering the serial number if you don't know the model number is seen at
http://configurator.memorystock.com/index.asp?classid=101&mfrid=77 The PCG-K33 model seen at the first link points at PC2100 DDR266 memory as seen at
http://configurator.memorystock.com/result.asp?MsClsID=101&MsClsName=&Mnfid=77&MsMnfName=&mn= VAIO+PCG%2DK33&modelID=48963 The one remaining question there would be whether that has two 256mb dimms installed. To be able to see a good 1gb of memory you then need two 512mb dimms.