xml versus string manipulation

I've got a situation where I need to hold more segregated information
in a database than there are columns available to me (I can't change
the database structure unfortunately).
At the moment I'm putting the extra information into an XML string and
saving that to the database; When it's needed I parse the results with
the XMLDocument object.
I've got a sneaking suspicion that this might be a bit processor
intensive ;)
Has anyone got any thoughts as to whether the gains made in just
manipulating the string outweigh the handiness of having a dom of the
data available?

ben