The new 320 Gb hard drive is in, Ubuntu is installed on it, and everything seems to be running smoothly.
Ubuntu immediately recognized the graphics card, informed me that the proper drivers weren't open source, and asked me if I wanted to install them anyway. After a bit of (ahem) discussion, both monitors are up and running happily.
My AIM account is working in Pidgin, the IM client that comes with Ubuntu; I've blown off ICQ entirely.
The data from my old D: drive is intact and fully accessible.
Now I just have to figure out how to install new software!