"display it on" is the wrong term.
You can take a signal (video, S-Video or composite or whatever) and Digitize it with an appropriate digitizer, and then send that captured DV data stream in the Firewire (or the USB) port of the iMac. Then, you can use a video editing program to display that DV data. There will be some lag between the input and the display, so it's likely not usable for games.
I haven't seen a VGA - to - Firewire convertor.
Please do a Search on the forums, we have covered this question about 20 times in the past year.