I've just done a quick n' dirty noise comparison of the a77, a900 and a99.
For my work - YMMV - the a77 is good to ISO 200 and OK at ISO 400. The a900 is good to ISO 400 and acceptable at ISO 800. The a99 continues this trend. ISO 1600 is good and 3200 is acceptable.
However, whereas detail begins to suffer with the earlier cameras at higher ISOs, the a99 seems to preserve it very well even beyond ISO 3200.
The camera is a winner!
(This is at ISO 3200 at 134%)