What if the brain wallet is just a memorable private key itself?
A series of random words (not a sentence!) long enough to use as a secure key should be about 12 words long, which could be around 100 characters long (figuring they're fairly long, 7-8 average, with a space in between). This gives you 2^128 combinations that you might have to try to crack your password, which might take about 1 billion billion years.

A private key, in base 58 format, is 51 characters long. Your idea of using a 51-character sentence as a private key would probably give about 51 bits of entropy (English is around 0.6 to 1.3 bits per character), which could be cracked in about 12.5 seconds.