I personally think starcraft2 looks beautiful with the iMac's 2560x1440 resolution. The current i7 iMacs can run starcraft2 with no problems at all, and same thing can be said for other prolific games. You also gotta keep in mind that RTS games depend majorly on the cpu compared to other games, (with reflections and battle effects) Don't be looking to run crysis at max or pad your gaming ego with fps numbers and how many times you can multisample though. Other than that, there might be some issues with later games down the road, but I'd say definitely get the iMac 27. At max resolution and max graphical settings, it can handle starcraft2 with the bare minimum fps to be smooth. If you ever find yourself getting below 24fps from time to time, then look to bring shaders down a notch for a dramatic increase in framerate with hardly any noticeable graphics loss.
Also do note it's only the beta, and blizzard might make the shaders more efficient in the actual released game or optimize it even more for os x.