Addition of a long sequence using PCR - HELP , PLEASE (Mar/24/2010 )

I want to use a PCR primer with a 30nt long sequence in the 5´ end that won´t be changed and it doesn´t bind to the target sequence.

I would like to know, which is the minimum length I should use in the 3´end to the primers could work property.

Design a primer as you would without the 5' tail. Then add the 5' tail, and check that there is not significant hairpin or primer-dimer problems as a result of adding the tail.