80 bits of entropy (20 really random hex chars) should be safe enough.

MD5 caps your overall complexity at 128 bits (realistically probably 110 bits with modern attacks). Typical english text has about 3 bits/character, completely random ascii is 7 bits/character (though that includes control codes you probably can't put in a password field). So 80 characters of typical English would be ~240 bits (obviously 80 characters of just 'aaa...' would be less), so definitely overkill for MD5; ~36 characters (110/3) is the point where you max out what you can do with MD5, and ~26 characters (80/3) is good enough. You can reduce these numbers a bit by using upper case, symbols etc., but personally I find it easier to remember 26 characters of normal English than 13 characters of random letters and symbols.