You could use
find /target/directory -type f | xargs perl -pi -e 's/stringtoreplace/replacement/g'
or if you are already in the right directory
find . -type f | xargs perl -pi -e 's/stringtoreplace/replacement/g'
and if you only want to replace in, say, html files:
find . -type f -name '*html' | xargs perl -pi -e 's/stringtoreplace/replacement/g'
You don't have to use perl of course, any tool that performs search and replace operations will do, and not doubt there are several available that are less resource hungry than starting perl (which is this case is a sledge-hammer to crack a nut, but it is an example I've had stored for ages and I've no reason to find a more efficient version). My original source for the "trick" is http://oreilly.com/pub/h/73
You can use all the options in which-ever tool you choose, so in this case the full power of Perl's regular expressions. The page linked to above has more examples, including how to search in a case in-sensitive manner.