maximum passphrase for symmetric encryption ?

[vedaal at nym.hush.com]

Is there a maximum size for a passphrase for symmetric encryption 
in gnupg, or does a passphrase exceeding a certain size not add any 
further security to the process?

Example,
The session key for AES 256 is 64 hexadecimal characters.

The approximate equivalent in brute force work is 20 diceware 
words.
[ 7776^19 < 2^256 < 7776^20 ].

 A string of 15 diceware words is often more than 64 characters.

Does increasing the passphrase string to more than 64 characters 
add any security?

Truecrypt full disk encryption insists on a maximum of 64 
characters for the passphrase.


(This is even more relevant in my case, where I routinely use 3DES 
;-)  )

(am not familiar enough with the primitives of symmetric encryption 
in how a string to key symmetric encryption works.)


TIA,

vedaal