This is a big question, real big.
It sounds like you need some sort of gateway that supports traffic prioritization, and a business class DMZ.
My personal pick would be a DFL-210, but you can call our sales number and they will take much more information about the issue and get you a better suggestion.
You could also toy with switch based QoS on that L2 switch of yours (if it is manageable, something tells me this shouldn't be on the DGS-1224T boards). That is however a bigger pain to set up correctly.
In short I would need more info, and this is not the best place to ask.