Its an awesome idea to challenge yourself in this way, if only that it makes you appreciate faster machines and more speed when and if you go back to them.
A few months ago I challenged myself to a week with this and only this machine:
Compaq Presario 1200 laptop
Pentium II 266 mhz
192 MB RAM
2 MB video RAM
6 GB HD
Tried several different OS's, settled on ConnochaetOS, a Arch Linux based distro designed for ancient machines like this. With it I had up to date mplayer, IceCat (firefox rebranded for a free OS), a minimalist webkit browser (XXXterm), abiword and gnumeric. I found that I could still watch youtube vids (3GP and 240p) if I downloaded the file in IceCat and played it back through mplayer with some command line tweaks. The machine was responsive if not fast. System used 30 MB of RAM when idle, I never seemed to run out of RAM even with several applications open. Shockingly, I didn't really miss my Dell D620 or ibook G4. In the end I gave away the laptop to a starving student who promptly sold it for 50 bucks cause she couldn't put itunes on it. Never again.