I ditched cable a long time ago and have been making do with Netflix, Hulu, and iTunes for my television needs. The displays on the new iMacs these days are big enough and high enough quality that I think they should be perfectly adequate for my gaming purposes, too. I will be upgrading to one of the new iMacs this fall, and I think it could replace my television entirely. Provided, that is, that I can hook up my Xbox 360 and my Wii, so I can play them on the iMac screen. Is this possible given?
What hardware and/or software will I need? If you don't have to upgrade to a new iMac, another option would be to buy a Mac Mini. You can use most any monitor you want with it, so that gives you many more options. If you buy a monitor that has both HDMI and DVI, you can use DVI for the Mac Mini (the Mac Mini ships with a HDMI-DVI adapter IIRC) and HDMI for your Xbox 360 for example. If the monitor also supports VGA, you could use that for your Wii.
Use the Displays pane of System Preferences on the connected Mac, or use the brightness keys on the keyboard of the connected Mac. To hear audio from a connected Mac using the built-in speakers of your iMac, select the iMac from the Sound pane of System Preferences. A Mac that's connected to an iMac in Target Display Mode can't use the iMac computer's built-in camera, its USB, ThunderBolt, or FireWire ports, or devices connected to those ports. To use external devices with your Mac, connect these devices directly to the Mac that you want to use them with.
This is the setup I use for my Xbox 360, and it works quite well. I have a Dell 22' monitor and it works great for this.