Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
3.5 inch desktop drives are thicker than your whole computer,

you can however, buy a Sata to USB, or Sata to firewire enclosure to put the 3.5inch drive into, and use it as an external hard drive. Enclosures aren't expensive, and provide plenty of cheap storage.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.