This EXCEL [attachment:fishr.xls spreadsheet] computes power comparing two group correlations using [:FAQ/WilliamsSPSS/Fisher:Fisher's transform] given the two group correlations and their sample sizes. This calculation may also be done [:FAQ/fishrpow: in R.]
The same results may also be obtained from [http://www.quantitativeskills.com/sisa/statistics/correl.htm this power calculator] although this calculator assumes the second correlation comes from a population, rather than a sample. In practice this means the second sample is thought of as having an arbitrarily large value e.g. 10000.