If a gigahash is 10^9 hashes per second, than 1 gigahash should be able to generate every possible hash of this in 1.4e4 seconds (or roughly 4 hours).
- snip -
Generating private/public key pairs involves Eliptic curve encryption. I'm not not sure either, but I suspect that it takes longer than SHA-256 Hash.
More importantly, anyone who is not re-using bitcoin addresses is protected behind ECDSA, SHA-256, and RIPEMD-160. This means that for each password that you try, you'll have to do the following steps:
Figure out what algorithm the user chose for converting a passphrase to a private key (typically just a SHA-256 hash of the passphrase).
Calculate the public key via ECDSA.
Calculate SHA-256 hash of the public key.
Calculate RIPEMD-160 of the resulting SHA-256 hash.
Compare the resulting RIPEMD-160 hash the a list of bitcoin addresses you are attempting to crack.