2.8GHZ on air to be honest is darned good.... Only chips I got so far on air stable with sane voltage on air (below 1.6v) with thermaltake typhoon so far has been 2.65ghz-2.7ghz. (and if you asked, this isn't bad at all considering heat dump this X2s are pulling off) (Dual prime 95 stable is what I mean when I mean stable)
That is 1 x2 4200, 2 x2 3800... Getting 4400 soon and will see how that runs.