hi there, i want to buy a new Mac Mini, and i want to use my iMac (late 2009) as display. I want to know if that's posible, and, if so, which cable should i buy
Mac mini
Only if it is the 27 inch model. See the following for cable to use. It is a mini displayport-mini displayport cable.
Use your iMac as a display with Target Display Mode - Apple Support
so i wont be able to use my iMac 21,5" nor my MacBook 15" (both late 2009)??
Correct. The 2009, 21 1/2 inch iMac will not work as a display. Not MacBook Pros works as a display via Target Display Mode.
You can use Apple's built-in Screen Sharing screen sharing to control/use one Mac from another Mac.
http://osxdaily.com/2012/10/10/remote-control-mac-screen-sharing-os-x/
If you do not have a fast network connection the lag will bother you.
iMac (late 2009) as display for Mac Mini
