Recommendation: Forget it. Upgrading from Sarge to Sid will probably break things majorly. Even an upgrade from Sarge to Etch is iffy because of the transition from Xfree86 to Xorg. You've seen how easy Debian is to install. It gets much easier with the
Etch Beta3 installer. Get the netinstal .iso (about 100MB), and do a fresh install of only what you want via ethernet. Coming from Ubuntu, you may prefer the GUI installer. Look for instructions to get to it from the initial boot screen. I've never used it myself, but folks are raving over it.