Moving a Site collection

Scenario:
You have a site collections running at http://severname:port/ on farm 1
and now want to move it to http://servername/sites/site on farm 2.

Required Permissions & Assumption:
Site collection Admin or Farm Admin account , Both contentDBs belong to same farm.

Command:

stsadm.exe -o mergecontentdbs
-url 
-sourcedatabasename 
-destinationdatabasename 
[-operation <1-3>
1 - Analyze (default)
2 - Full Database Merge
3 - Read from file]
[-filename ]

stsadm -o enumcontentdbs -url 
stsadm -o enumsites -url  -databasename  

Example Usage:
Best way to use this command is to generate the file
by using the stsadm -o enumsites command. Open the file and remove the lines within the site collections you do not want to move. Save the file and use it as shown below.

stsadm.exe -o mergecontentdbs -url http://server_name
-sourcedatabasename WSS_Content_SOURCE_DB
-destinationdatabasename WSS_Content_DEST_DB -operation 3 -filename listofsites.txt

Related Articles:
http://www.toddklindt.com/blog/Lists/Posts/Post.aspx?ID=53