Apple has updated the selection of available storage devices on the iMac product. The new options are a 256GB SSD and 512GB SSD. Previously only a 5,400RPM 1TB drive or a 1TB Fusion Drive was available. The new options cost an additional $300 and $600, respectively, but should bring a noticeable bump to performance.
These additional choices mostly effect the 21.5-inch iMac. The 27-inch iMac did already have a 768GB SSD option available, though it came in at a costly $900. Some users don't require mass amounts of storage and only require speed. Apple is now catering to those users with some reasonable SSD choices.