I doubt that Comcast will pull someone's internet if they "use it too much". My father is Comcast's sales manager for the state of New Mexico. I asked him this question, and here's what he told me.
Comcast's internet use policy prohibits using the service for running servers for web hosting and constant file transfers. They recommend the use of third party remote servers for those purposes. It is true that Comcast can and will disconnect the internet for people that overuse the service, but it is to prevent this rule from being broken. My household uses the internet on a daily basis for several hours a day. I use XBox Live for at least one hour a day. This brings us nowhere near the maximum. In fact, my dad tells me they only had to disconnect one person in the entire state in the last two years, because he WAS doing insane amounts of data transfer from a home based server. It taks a LOT of constant data transfers to make a disconnect happen.
Therefore, unless your mom is illegally running a home-based internet server, there will be no disconnection.
My own personal web hosting and email is done through
http://www.surpasshosting.com, and not from a home owned server. Therefore, when you access my website, the transfer rate is only going to the remote Surpass Hosting server and not through my Comcast home internet. There are some top rate T1 companies that will allow home operated servers, but all cable companies do not provide that.
As for connection stability, it is based on the fact that Cable internet is shared with other households (through area "Nodes"). If there are a lot of people using the internet at the same time in your area, the internet can slow down. This is true for any internet provider, but the cable line is used more constantly by more than just internet access. So, slowdowns are more common. It's great for XBox Live, but if I ran a home buisness, I'd go for SDSL (Symetrical DSL) for higher upload speeds.
Lurking softly, reading your posts, loving your ideas...
-Kaebora