Sounds like you have one of the newer line of processors, either a 1.67GHz Celeron M, Pentium Dual-Core, or Core (2) Duo. If that is the case, then they do not make 2.8GHz models of those processors, except for the Core 2 Duo X7900 which cost several times more then your laptop itself.
Point is, the GHz rating scheme doesn't allow you to compare newer and older processor series. So your 1.67GHz processor may be faster then a 2.8GHz older style Pentium 4.