There is a rollbar and there is a rollcage.
The determination of which you need depends on the class you will be participating in or the Elapsed Time or Trap Speed you will attain. As a for-instance, a minimum 5-point rollbar with 5-point (SFI 16.1) safety belts is required at 11.49 or quicker. A minimum 6-point rollcage is required at 10.99 if the floor of the vehicle has been modified in excess of 4 square feet (not including tubs) or if the firewall has been altered in excess of 1 square foot. If the floor and/or firewall are unaltered beyond the figures I listed here, then a rollbar is legal to 10.00 E.T. and 134 mph.
A minimum 6-point rollcage is required on all vehicles at 9.99 or quicker and on all vehicles that go 135 mph or faster in the quarter mile.
Pony up 10 bucks and get yourself a NHRA Rulebook.....
Summit Racing® NHRA 2012 Rule Book SUM-NHRARB - SummitRacing.com