I'd look at getting two different cards. One PCI-E which will drive your primary and secondary displays and then get a PCI (with a composite out) for your stage display. Should work perfectly for you.
I was just looking at the iMac line and I'm really not impressed. I mean, you'll be paying 2x as much for a core i5 iMac compared to a similarly spec'd PC. Also, iMacs don't have eSATA interfaces for some reason. You do have firewire 800 but it's not the same.
I guess this is more your preference. We've got a copule macs at our church but our number crunching video editing computer is actually an 8 core xeon computer. Would we like a mac pro, yep. Is it worth the money...in my opinion it's not.
I'd rather you buy a decent quality PC, get either an Avid or Premiere editing suite. Stick a blackmagic intensity card and you'll be good to go. Maybe you can get 2 TB scratch drive and a 2TB drive for storing rendered projects. Keep a 1 TB for OS/Programs.
Derek Van Winkle
FBC Biloxi, MS