There are video cards with Tv-Out (I'm partial to the ati radeon's, but most manufacturers offer a card with tv-out), and these will typically have a 15-pin monitor output and also a RCA/S-video output to go to your TV. usually you can run both at the same time, either to show the same image on both screens, or to make your desktop twice the size(ie 1600x600 for example) spanning both monitors. Or you can just use the TV out, to use your tv as your only monitor.
The setup is pretty straightforward once you have the card installed, it's just some settings under the Display settings in windows
Drawbacks... Hmm only one I can think of is that if you start using it for mame, you might never go back to a regular monitor. Seriously though, if you intend to use a TV as your only monitor, the clarity usually just isn't there when using Windows, text can get fuzzy, unless you run at a low-res, like 640x480. A dual monitor setup is pretty sweet when youve got games and videos going to the TV and your regular windows on the monitor