A Practical Introduction to GNU Privacy Guard in Windows recommends DSA and ElGamal, but I would like to know if RSA is good enough to use these days, and if so, what minimum key size should I use? Is it ok to use SHA-256 for signing (for compatibility with e-mail clients)?
Also, beside e-ignite: Key Types, can you point to other sources for this subject?
RSA/DSA minimum today is 1024 bit actually, so Elleptical Curves becoming more in use since they are faster and using shorter keys.
To have a similar security as AES256 you will need at least 3072 bit (384 bytes) key...
Email clients using certificates nowadays - so it's separate thing (X.509), but for using with RSA/DSA most common option is SHA-1 (somewhat weak now).
I recommend study of: