Total waste of money. On that old banger you'll get, at best about 10-15 bhp, an increase which you simply won't notice and as for better mpg, just run it on the highest octane Shell V-Power [or BP Ultimate or the Esso version] which should give you 20 mpg average instead of 18.
Far better you get yourself a cold beer, sit down and take time to read the buying a used Touareg thread - the link is in my signature below.
You'll learn a lot, know what failures are coming your way, and probably save money too.