Since I have my Sony Alpha 33 I was trying to use High ISO but I was clearly not pleased over 400 ISO (in dark condition else it's OK).
As Sony tool was not enaugh good to treat High ISO RAW I baught DxO 7 : this tool is fantastic ! Thanks to it I'm now able to go to 1600 ISO very often.
Anyway over 1600 ISO, quality was not good. Today I think I have found the solution. If you underexpose with IGH ISO, the resulte is really bad. If you over expose, it's perfect ! I did a lot of tests and the result is always the same. Now I can use 6400 ISO without problem. Try it (but it may work only with A33 ?)!