I'm not a Unity fan, so I'll pass on Ubuntu. But I did set up a Mint install a couple months ago. 25 - 30 of my Steam games run on Linux, which is about 20-25%. More than I expected, and including some good ones, too. Since then, I've been going back and forth between XP x64 and Mint, but it's still probably 85% in XP since more of my programs run there than in Linux.
I have a Radeon GPU, and once I switched to the proprietary drivers, they've been all right. The open source ones were impressively poor, though. Some games have rather poor OpenGL support, but others are just fine. Haven't tried Civ5 since I prefer 3 and 4 to it. So all my Civ time in in Windows.
Day to day, I don't find any great difference between XP and Mint - they're both pretty good at staying out of the way, they're both stable, and they're both sufficiently lightweight with adequate performance.