Gears of War PC
And it's got an updated campaign mode, new boss fight, and probably lots of other neat stuff (like maybe less lag online with the addition of dedicated servers). I hope this stuff comes to the 360 as downloadable content (FREE, downloadable content...the Epic way, not the Microsoft way).
But I'm not regretting my trade. My PC wouldn't have been able to handle Gears of War. It couldn't even handle GRAW.