The interface standard speeds are 1, 10 and 40 Gb/s. I;m not aware of any interfaces that run at 20Gb/s.
If you're looking for an IPS sensor that can process 20 Gb/s of traffic:
SourceFire has the 3D8260 (their clusters run 20, 30 and 40 Gb/s)
McAfee is releasing a new line this summer but I can't remember the model number off hand.
Alternately you can statefully load balance across several sensors (clustering).
You will soon discover going fast is damn expensive.
- Bob