I'm not a real gamer, but I have been involved with Civilization 3 and 4, and most of the issues I listed have applied to them at various times.
I have Civ3 Conquests for both platforms, and it plays very well in Parallels, even though its read-me specifies DirectX 9b hardware support. It is only a 2D game though. I'd say it's at least as fast as the most recent Universal Binary from Aspyr, plus you get a full scenario editor. The only downside I can see is there are a few keyboard shortcuts that don't seem to make it through Parallels and you have to use the menu options.
It will be very interesting to see if Civ4 runs in Parallels. I don't have Civ4 for Windows, as I refuse to reboot just to play a game. I'm told the Windows version significantly out-performs the Mac port in Bootcamp on the low-end Intel Macs with integrated graphics. But it does demand very good 3D graphics support, so the jury's out on whether Parallels will support it. If anyone cares to share their Civ4 experience under virtualization I'd be interested to hear it.