If you have the 500 already and you have to buy a controller either way, you may as well get the EZ boom and use the 500 for your controller.  

I would probably skip the foamer, take that money and add a little more to it and add EZ steer to the deal while you are at it.  Once you have the EZ Steer,  you won't want to go to the field unless you have it in the machine you are using.  My figures say that assuming a machine which is worth $100 per hour to run, you only need to run about 100 hours per year in order to pay for the steering just from the cost of the machine time spent working ground which is already worked.  If you add in some application passes where you are wasting inputs as well as time, it is a complete no-brainer.   NH3 will pay for it in a couple thousand acres...I mean pay for it outright, not just pay the cost for the year.