multiple / different GPUs in same machine ?
I've currently got 4 R280's (7970's actually) in my server, and am looking at adding 3 more into the same rig - will adding 3 R290's give me the extra cycles per card that I'd expect, or should I stick with the same platform?

I guess what I really mean is - is the load shared equally between cards, or do cards "draw" more work as they finish processing their individual queue?
mixing and matching normally isn't a problem, but in this case you're talking about mixing Overdrive5 / PowerTune 1.0 and Overdrive6 / PowerTune 2.0 cards, and at least one person has reported problems with this combination. you can likely make it work as long as you run with '--gpu-temp-disable --powertune-disable' and manually manage your cards, but sticking with 280s you shouldn't have a problem.