Just had an iMAC 27' for the past 2 weeks:
It has the 4670 chip.
2560x1440 is simply TOO much IMHO for any GPU.
1- Sims 3...ok at 1600x1200
2- Star Wars Force Unleashed..struggles at higher resolutions, very playable at 1280x800...or something like that.
Older games...like GTA, Unreal tournament, Bioshock etc...All run fine but NO WAY at 2560x1440. More like 1680x1050 MAX.