5DIII + 24-105/4L will deliver noticeably better low-light/high ISO IQ than 7D + 17-55/2.8. For low light candids, I also agree that an f/2.8 or faster lens is the way to go.
Personally, I prefer not to depend on IS in that situation - even if the camera is stable at 1/30 s (IS or tripod), subject motion is a problem, and especially in candids where subjects aren't 'holding still' for the camera (even in posed shots, I prefer at least 1/60 s). On my 1D X, I use a minimum 1/125 s shutter for shooting people - and I let the ISO run up to 12800 with no worries.
+1 neuro. don't depend on IS either. anyway, your 1Dx surely must deliever excellent IQ beyond the 12800. I know for reasonable noisefree nighshots the 5D's limit is at about 5000 or 8000. There was a dude earlier this year who came up with a comparison to the 1Dx.
Two questions for neuro:
1) Did you take an ISO 204800 or 102400 photograph? I tried one at 102400, exposing well to the right (not a very dark room) and outcome was pretty well although it required quite some NR to make it look right.
102k no NR
Z96A0008aBWkNR ISO102400 no noisereduction by Peter Hauri, on Flickr
Z96A0008aBWUsMaNR ISO102400 by Peter Hauri, on Flickr
2)What do you think can be expected from a 5DIV or 5DV (whatever they may call it) at hopefully about the same MP with an improved sensor design as the rumors are floating around? Will we see 51K or 102K IQ compared to what is now 25k IQ for the 5D series? Or let's say, will IQ at 102k improve at least half a stop or a as up to a whole stop in RAW until 2015 or 18? Thanks for your assessment on this according to your profound technical knowledge...