I recently built an HTPC for Media Center built around an i3. I threw in a GTS 450 for $60 and I got a cheap gaming machine. First of all gaming on over 50 inches is great. But the sweetest part is hooking it up to a 5.1 Surround Sound System. Then it really starts rocking. Plus a couch is far more comfortable over a chair.
Only downside is it's a pain to use a keyboard and mouse so I use a 360 controller instead. Which is not a good idea for Multiplayer gaming mind you. But for console ports it especially works well.
I recommend everyone to try connecting their main/gaming pc to the home entertainment system.
I have mine hooked up to a 42" LED monitor - it's very amazing. I don't have a fancy sound system or anything yet - but the Klipsch speakers that I've been using more than make up for lack of surround sound.