Kundan Lal Rana :
if you are using QuickBooks pro, 2.0 GHz processor is recommended. avenseth12 is right.
In that case, a 2GHz Celeron from 2002 should work fine. Speed doesn't translate across different architectures that well, so 2GHz is too broad of a term to use.
QuickBooks is business accounting software. Anything will run it.