Comparing the 5D2 RAWs to the 5D3 RAWs in ACR and Photoshop it looks like the 5D3 is two-thirds of a stop better for high ISO SNR.
Lots of dangers in the comparison though since we don't know what ACR does to them for each camera or even if the ISO stops are rated the same way or not.
We really need DxO for the SNR measurements.
But looking at these, I'd say a solid 2/3 stop advantage for 5D3 over 5D2 SNR.
At ISO 100 it seems the 5D3 won't do much for you, early reports are basically no increase in dynamic range and only modestly better banding (still likely worse than the 1Ds3 at ISO 100). The dynamic range tests were carried out using the masking region though so there is a modest chance that they might not fully tell the true story at all, although they probably do.
Based on all of this, sometimes a bit sketchy info, the 5D3 may offer no advantage over the 5D2 at low ISO but a solid 2/3rds of a stop advantage at high ISO (almost D3s-level, but the greater MP actually makes it effectively better than the D3s since you can NR more and still have the same final detail*).
(The D800 would offer 50% more MP and perhaps a solid 2.5 stops better dynamic range at ISO 100 and perhaps between 1/3 stop worse to 1 stop better performance at high ISO. Based on that, the D800 most likely does have an all around better sensor, although it is not set in stone yet.*)
* note how despite all the talk about how high MP cams make a mess of things, the ones with higher MP don't seem to be doing so badly compared to one with lower MP