I think it might a bit in CPU intensive games, in fact I'm almost positive there will be a slight bottleneck, but in more GPU intensive games you won't see much of a negative impact.
OK, well I don't play any FPS games. Mostly racing and RTS. Civ 5 is a very processor-demanding game, what kind of bottleneck will I be looking at? As in, approximate FPS drop on a 20" 1600x900?