I remember a site that use to tell how you can solder a memory card (compact flash, secure digital...) onto certain points of the ps2 memory card but I can't quite remember where it is. If you happen to find it please post because I have a cf card here and would like more space myself. However if you don't wanna crack open your ps2 memory card there are things you can get that is basicly the same thing you just plug a ps2 memory card into it and then 8 megs + however much you bought (16 and 32).
anyway here's the site for them
http://www.gocybershop.ca/ just go into Playstation 2 -> Memory cards and hten it's PS2 Expansion Memory Card xx Megs