Can I use my early 2009 iMac as an external screen?
The specifications for the 24" Early 2009 iMacs according to eveymac.com and the Apple Manual for the Early 2009 series of iMacs does not mention target display mode as a feature.  That unfortunately means that the Early 2009 generation of iMacs did not offer a target display mode so you will not be able to use that displayport output as an input source.