A very basic question, but something that I haven't really thought about before. What are the performance hits that affect flash video and streaming in general?

I'm asking for two reasons. First, my wife loves XM but for some reason, on her machine she has frequent problems with the stream stuttering and sometimes locking up entirely. On my computer, I can't make flash video full screen or the framerate drops at least 50%. I really like Hulu, but I can't watch it in a little window.

Normally I would assume the usual suspects, but we have FIOS with a steady 5mbps down, so I would assume streaming music would be no problem, and my machine, while a bit old, is still a 3.2GHz system with a gig of memory. Besides, neither machine is working too hard when performing either of these tasks. If I've already buffered the video I don't see why a large resolution should kill the framerate. Is that just a limitation of flash video?