Would you get much for your 500 DG Super? If not then you should keep it and experiment with multiple lighting off camera setup

That's what i did! First of all I started with the 500DG Super as a slave. The 580EXii can control the 500DG Super!

That was fun but i wanted two off camera flashes. At this point i had one of two choices. Either to use the radio triggers (which would involve losing ETTL-ii and doing things manually) or buying the canon infra red trigger (which maintains ETTL-ii). I went the latter route and am very happy.
The interesting
thing is that my 50d couldn't talk ETTLii with the 500DG super (i originally bought the flash with my 300d and ettl worked fine but they changed something in the 50d). However, the infra red trigger and the 580exii both seem to be able to drive the sigma flash just fine
Just a thought...