Most browsers have supported at least one 256bit cipher since 2006-2008. You can't assume 256bit for the general public yet (though you'd be blocking out a very small minority at this point), but you can safely assume 128bit is supported these days.
Also, there are 4 different 128-Bit ciphers that are commonly implemented (RC4, 3DES, AES, and Camellia). Each has pros and cons. RC4 is the fastest, and probably least secure. 3DES has problems up and down, and should be disabled in my opinion. AES and Camellia are fairly secure, though FIPS requires AES currently.