Changing signature algorithms

[MindFuq]

I'm using GPG version 1.0.7 (stock), which is capable of the following
algorithms:

  Cipher: 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH
  Pubkey: RSA, RSA-E, RSA-S, ELG-E, DSA, ELG
  Hash: MD5, SHA1, RIPEMD160 

My problem is that I can't find the switch to create a non-SHA1 hash.
I want to sign using MD5 or RipeMD160.

Another problem is that with the --cipher-algo switch, I cannot change
the algorithm used to encrypt the hash on my signature.  In fact, this
switch won't even let me select the default DSA algorithm.

I'm guessing that the answer to my second question is that the
algorithm used to encrypt the hash is determined by my key.  Is that
correct?  If so, then what is the --cipher-algo switch used for?

Then I tried to generate an RSA key, and was told it could only be
used to sign messages.  Why can't it be used for encryption?  To
confuse things, I ran a 'showpref' on the key, and it did not list any
asymmetric ciphers, not even RSA; but it did list symmetric ciphers:
AES, CAST5, 3DES.  How are these ciphers used with a key that can only
sign?  My understanding of a signature is that the hash is encrypted
with an asymmetric key, not a symmetric key.