The String object is all fine and dandy to work with when you are on a machine with plenty of memory available, but not on these little chips. You're better off ditching trying to store them as Strings (or strings for that matter) and simply extract the values from each request.
strtok() can be used to iterate through each part of the transmission separated by a ';'.
strncmp() can be used to check each deliminated section to determine if it's telling you a a node or a value.
atoi() can be used to convert the the string number int an int.
And when you're ready to send the data back out as a string...
sprintf() can be used to construct a formatted string containing the data.
Although if you're hell bent on not storing them as ints, you can simply have an array of strings (note the lowercase S) and use strncmp() to determine which node the new transmission should replace.