If you want to move some site collections from one web application to another which mostly happen when you have huge number of mysites and finally you have decoded to move to new content db.  You have to use 3 command together with configuring mysite.

STSADM –o enumsites

STSADM –o enumcontentdbs

STSADM –o mergecontentdbs

Look at the articles given below to learn quickly.  Listed below are by quality.

Move Site Collections in a Single Bound

Ensuring that My Sites are used in a compliant manner.

MOSS 2007 – How to move MySites from one Web Application and Content Database to another.