As a very rough guide I always say approximately for a 2wd car:

200bhp- 100mph terminal
300bhp - 110mph terminal
400bhp - 120mph terminal
500bhp - 128mph terminal
600bhp - 135mph terminal
700bhp - 140mph terminal

I know there are all kind's of exceptions but that is a good general guide for a normal road going 2wd car.