It depends on which games you play. The 6770M iMac is about double the performance of the 6490M MBP and comparable price but the MBP will still play most games at a decent quality and framerate.
Also consider the Mac Mini with the 6630M GPU. It's only $800 (almost $1000 less than the others), is faster than the 6490M MBP and you can plug it into a TV via HDMI (1080p 32" would be about $300-400) or you can get an IPS screen for about $200-300 (20"-23").
It may be a little sacrilegious but I'm thinking that a for a dedicated gaming rig the best mac for gaming might be a hackintosh or simply just using Win7 and skipping the OSX aspect entirely.
Many mac ports IMHO have been total suckage in comparison to their PC version. If you have a 2011 MBP and you want a dedicated gaming rig, that'd be my recommendation.
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Also consider the Mac Mini with the 6630M GPU. It's only $800 (almost $1000 less than the others), is faster than the 6490M MBP and you can plug it into a TV via HDMI (1080p 32" would be about $300-400) or you can get an IPS screen for about $200-300 (20"-23").
Many mac ports IMHO have been total suckage in comparison to their PC version. If you have a 2011 MBP and you want a dedicated gaming rig, that'd be my recommendation.
Or just a PS3 if you don't already have one.