Hi, Todd!
I recently did the same thing to my WallStreet. Piece of cake! I bought my 256 Mb memory module from OWC. I put mine in the upper slot so I would not have to remove the motherboard.
At the same time, I replaced the original 2 gig HD with an IBM TravelStar 20 gig, also from OWC. No problem!
I already had a USB PC card installed, so I bought a USB enclosure (OWC again) so I can use my 2 gig drive externally. Works great.
Kelly's links should tell you all you need to know about the procedure.
Best regards!