How to wipe a thoroughly secured old computer
You can remove the firmware password by changing the RAM configuration, booting up to the startup menu, shutting down and changing it back. Basically, if you have two 1GB chips of RAM in there now, you can remove one of the chips, start up holding alt/option, shut down and put the other chip back in. This will remove the firmware password. 

Now, you can follow these instructions for doing the erase and install.

Good luck!