Netflix is in the 500MB to 1GB range for a show I think. Haven't paid much attention since my cap here is 250GB.
As for multiplayer gaming, 64 kbps is a good, high number as an estimate for data usage while gaming. Patches etc. chew up more bandwidth, however gaming itself uses next to nothing.
As for computer updates, averaging things out, I'd put monthly usage at around 500MB for patches etc. For appliances, the number is probably closer to 100MB, if that.
For all the whining everyone does about low caps, let me give you a scenario. Three members of my immediate family (dad, mom, littlest brother) share a 1.5/384 DSL connection. E-mail, web, some YouTube, a movie on Netflix once every week or two (standard-def), some internet radio, a fair amount of online gaming (Flash or full-on multiplayer). Three Windows computers, plus a smartphone. Every couple of weekends my other brother comes home, with his laptop and smartphone. He adds to the YouTubing, gaming and update-downloading. Every month or two I come home for a few days, and add my own update downloading, YouTubing, heavy web surfing, Pandora, online gaming and file-shuttling (in addition to photo uploading that my mom does), on a smartphone, a laptop or two and a tablet.
Total usage? Last I checked, under 25GB, despite the fact that we don't make it a point to watch our bandwidth usage, because the connection is unlimited, on the data transfer front anyway.
The nice thing about being in a rural area is there are other things to do than consume content online.
Don't get me wrong. If you beat the heck out of an internet connection, you can certainly make it above 25GB in a month. Case in point: my Comcast usage numbers (I upgraded from 25/4 to 50/15 this month):
March: 271GB (and counting)
But that's due to using the connection for every possible use case (work or not), on the following device set:
Smartphones/iPod touches - ~7
Tablets - 2
Gaming consoles - 3
Computers - 8+
I blame Netflix for 75 of those gigabytes...and I wasn't even watching the Netflix'd content part of the time