I have the Comcast Blast package (fastest one they offer and work pays for it!) and it only streams when someone is connected. On the more popular days, when I message my friends and family that the dogs are on the "puppycam" it only uses about 2-4GB.
The 28 gallon camera is a logitech 9000 ($100+) or something like that and the other one is a logitech 5500(~$50). They are both USB. My place is so small that my tanks are pretty much next to my laptop and then router. The logitech 9000 has auto white balance, auto focus, auto brightness which is cool and all but it doesn't make that big of a difference. The 5500 doesn't have any of those features and has less megapixels.
Interestingly enough, I set both camera's to run at less than 275kbps and my wireless network just could not handle it wireless..ly.. It was very choppy and always buffering. I have a really nice wireless access point and still it had a hard time. Once I hardwired the laptop in the buffering issues went away. The most people ever connected at once was in the 30-40 range when I did a little first time zoa fragging for the people over at nano-reef live over the webcam and of the people I asked the quality was great even with the 2 camera's.
Check out the app "Active Web cam", it's a solid, cheap, much better than the other ones app that will stream for you and fairly easy to use. I tired a myriad of software out before settling with this one, it was the best bang for the buck and the best quality.