using an imac as a monitor
Can you use an imac as a monitor if you get a new mac computer?
iMac, OS X Mountain Lion (10.8.2)
Welcome to the Apple Support Communities
You can use an iMac as an external display if it's a 27-inch iMac or an iMac with Thunderbolt ports (Mid 2011 or newer iMacs) > http://support.apple.com/kb/PH11302
If you haven't a got a 27-inch iMac or an iMac with Thunderbolt, you have to use a VNC client that works over your network, as ScreenRecycler, to use the iMac as an external display. However, these type of applications may be slow