the $1200 one is a complete system, if you add it to your basket you can modify the bits it comes with and bring the price down (you don't need a DVD Burner for instance, downgrading to a DVD/CD-R combo drive saves you a bit) The ~$500 one is a bare bones unit that comes with case, motherboard and Powersupply I think, it's all in the item details. You just need to add the RAM, hard drive and slim line optical drive. (it already has the riser cards installed by memory too, but best to check with the seller. send a pm to Steve ;) he'll change it...