Network Bandwidth monitoring

I plan to incorporate my MRTG system into Xymon.  I would like to know
if I can have a "Red" alert appear when my bandwidth utilization of a
particular interface exceeds a certain amount over the 5min interval?

Is this possible?

If so, can you point me in the right direction so I can research how to
set it up?

 
I have a DSL circuit I need to alert when it reaches the 500Kb upload
rate.

Thanks,
Mike