dbcc traceon(3604)

Usage: 3604 is to redirect the output onto the screen/file. default is to
direct the output of the dbcc's that come after 3604 is errorlog. and is
session based.

impact: depending on what dbcc your are planning to run will impact the
system.

if you use dbccdb, impact is minimal.

hope someone else can shed more light on this.
Vipra.