Your potential iMac will be able to run Civ5 via BootCamp and Windows. My recently acquired refurb 27" i7 will run it. (Don't tell the wife, but the upcoming release is one of the reasons we needed to get the iMac).
Fortunately for me, I couldn't care less about running with all the graphics settings maxed. This game doesn't need the eye candy to be good -- in fact, in some cases it distracts from the flow of gameplay. There were tonnes of complaints to this effect when CivIV came out, and the community was frustrated that developers had focused on detailed visuals, stressing the GPUs of older machines. As a turn-based strategy game, the whole series is based around highly complex math computations happening thousands of times per turn. If anything, the stress should be on the processor.
But I digress. Again, you will be able to run Civ5 via BootCamp. On your new iMac, likely on high settings or better. But you'll have to decide for yourself what's bearable. Finally, I fully expect we'll see a Mac version down the line, if you want to hold off for it.