I know it's a sin, but I'm getting some computer parts off a friend who's doing some upgrades and wanted to put together a gaming PC for myself. Seeing as how monitors these days are costing about $200-ish, I was wondering how I'd be able to hook my iMac up to use as a monitor for the PC. I've been reading about adapters and stuff, but being a not-so-tech-savvy person, I thought it was best to seek help from experts.
As far as I'm aware this is impossible - the iMacs can output video but cannot take a video input.
There are I believe some kludges you can do - if you can get your video stream output via FireWire or ethernet, then you should be able to find solutions that will let you display that video stream on the iMac, but that isn't really using the iMac as a monitor (and if you want a gaming PC the latency would presumably make it completely unsuitable).
the iMacs can output video but cannot take a video input
The current 27" iMacs actually can be used as a monitor. [apple.com...] :
The Mini DisplayPort lets you connect an external display, including the Apple LED Cinema Display, to your iMac. On the 27-inch iMac, the same port offers input, too. So you can connect any external source that has DisplayPort output — including a MacBook or MacBook Pro — and use your iMac as a display.