Try something like:
while read name
do
sed 's/OldString/NewString/g' $name > NewFile
mv NewFile $name
done < list_of_files.txt
where list_of_files.txt contains the names of the files to be searched and changed. I believe gnu sed has a facility for in-place editing, but perhaps Feherke, if he's around, can advise further. HTH.