I have a site which is a white label (Multiple versions of the same site) which I\'ve launched recently. There isn\'t a great deal of traffic yet - mainly bots but probably 800
It does sound like you are on the right track in looking at this dm_exec_requests DMV. I suspect you have already seen this, but there is a fair bit more information on the 180 throttle limit which is documented here and outlines some key reasons for it.
If it is of interest to you we have a service called Cotega which might be helpful for both of your questions. The first is that we can run all the key DMV's against your database to show you what is happening to help you analyze your db and we can also notify you (email, sms) when you start to get close to your throttling limits.