The Fusion drive is a $250 build-to-order option for iMacs and Mac minis. Apple doesn’t push this option very hard, but it’s something you should strongly consider. The Fusion option gets you a 128 GB solid state drive in addition to the standard (mechanical) hard disk drive. The read speeds on the SSD are in excess of 400 MB per second. That’s very fast – several times faster than a mechanical drive. An iMac with a Fusion drive will boot in less than 15 seconds. Most applications will load almost instantly. Larger applications can still take a few seconds to load. However, you’ll definitely notice how much snappier a Mac with a Fusion drive feels.
Since all your data won’t fit on the SSD, OS X will intelligently move your files between the SSD and HDD. The most frequently accessed files and applications are kept on the SSD.