No. A Mac (or a PC or laptop for that matter) doesn't have video input. Sometimes you can add a TV card (so it has an input for analog or digital video, like your cable company supplies), but then you'll see that your new iMac can't supply such an analog or digital signal on a coax cable.
I would like to use an older iMac G5 with the PowerPC chip as a second monitor for my new iMac 27" G5, I this possible and if so, how do I go about it
Thanks in advance for your help