a lot depends on your area and the type of broadband your using.
any heavily populated area will have peak times of usage that will slow everyone down using the net. this is regardless of the type of broadband or the providers claims. (even FIOS)
I stream music 24/7, literally, and my net doesnt slow down, but Im not in a heavily populated area. I do see random delays/lag on occasion, but that happens regardless of what Im doing. the only thing I have noticed is the "cache" can become quite large, which will create a slow down on your system itself. and that will happen no matter how much of a beast your system is, that is due to your OS.
2k3.5 Protege5 - - stock DD
1993 MX-6 LS -- still a work in progress....*black beauty*