Apple's pre-configured iMacs currently ship with 1TB hard drives, but you can now have 256GB or 512GB flash versions if you are willing to get a custom one built. They are priced at $300 and $600 over the original price.
The 768GB flash option remains available for the 27-inc model which will set you back an additional $900. Of course discounts are still available for corporate and educational establishments.
Source: Apple Store