Question on Bush's secret wiretapping order

President Bush claims, "To save American lives we must be able to act fast and to detect these conversations so we can prevent new attacks" -- CNN

The question is how how many secret wiretaps has the NSA done and how many terrorist attacks have they ACTUALLY prevented because of this order? It's been four years. What kind of results has the NSA shown for the grant of additional power? My guess is that the NSA took this opportunity to gain more power for themselves so they can conduct their own private operations. If they were actually spending time cracking down on terrorists, how many have they caught so far due to this law?