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My 2011 entry lvl 21" handles games ok , albeit older MMO's like City of Heroes and WoW.
Downloaded a trial for Rift , a newer MMO which i'm running on bootcamp again with no problems on medium to high settings.

Suppose it depends on the games your after. One thing i do notice it when game playing it heats up real quick to such a heat i can't really touch the top of the machine :) , i've been told this is ok and usual but i do get worried now and then.
 
...it also heavily depends on your demands. What some people would regard as "runs like a dream", others would say "yeah, a nightmare".

The top end 27" would, obviously, run contemporary games the best but if it is good enough is up to you - it is by no means slow but it is far from optimal for gaming.
 
Depends on the iMac.
The top-end 27" runs games like a dream.
This is true. As a proud owner of the highest end 2011 iMac 27", I can testify that once you install Windows 7, this thing plays all my games at max or near-max graphics settings at very respectable frame rates.
 
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