Well are you hard up for cash at this point? if so I would put in another stick of RAM maybe 512. What speed can your mobo handle? If you have a significant amount of cash avaliable I'd go for another computer.
Newegg.com - TOSHIBA Satellite P105-S2036 NoteBook Intel Core Duo T2050(1.60GHz) 17.0" Wide XGA+ 1GB DDR2 533 80GB 5400rpm DVD/CD-RW Combo Intel GMA950 - Retail
Upgrading the HDD would only speed up the computer (^^remember to defrag^^) depending on the speed and cache of the upgrade, like TheEnd said, but I doubt it's slower.