How can I observe memory bandwidth?

There is a memory bandwidth benchmark available in open source. It works for Intel & ARM under Linux or Windows Mobile CE.

It will give you raw performance for your memory as well as system performance with memory. But it won't give you a real-time bandwidth, so I don't know if it's a good answer to your question.

There's also a memtop tool out there, but it's more about usage than bandwidth. Perf tool can be handy in order to detect page fault.