I noticed that Debian had released version 5.0 Lenny. Considering I was still running my server on Debian 3.1 and had completely missed 4.0, I decided that it was probably time to upgrade.
I went to Debian's site, followed the handy link to the release notes (that was in the sentence "If you are upgrading from a previous version"), went to the chapter about upgrading (chapter 4, if you were wondering), and followed the step-by-step instructions.
When I rebooted my system, I did so directly from the machine itself, which was good, because installing the new kernel did not select it as the default in grub. I also noticed that iptables did not load its rule set, and I quickly discovered it was because I had recompiled the previous kernel myself to include a "TARPIT" rule that didn't exist in a stock kernel image. Updating the rules to just go to a "drop" rule seemed to work, except my wireless subnet was having difficulty connecting. Long story short, I didn't need the extra drop rules (basic ones already existed), and once I removed them, all was right with the world.
I was even able to upgrade PHP to version 5 and finally get the graphs working in the vnstat PHP front-end. (They're still limited to the rolling 30 days that vnstat records, so I'll still be using my own graphs for my reports, though.)
All in all, it was a pretty painless process, most of it I was able to do from the comfort of my laptop.
 Um, mostly. It seems that about a third of my packets between the internet and my wireless subnet are just getting dropped. Figures; it couldn't be that smooth.