Which CPU for gaming laptop

Good morning, am about to order an alienware m15x gaming laptop but am unsure which cpu to spec it with. It will have 4gb ram and a nVidia GTX260M but cant work out whether to go for an Intel® Core™ i7 Processor 720QM (1.60Ghz, 6MB L3 cache) or Intel® Core™ i7 Processor 620M(2.66GHz,4MB cache). The 620M is an additional £120 but if the performance increase is going to be noticable I will happily pay it. The 820QM is out of my price range so not an option. The laptop is going to be used only for games and will always be plugged in so not worried about power consumption. The games that will initially be played on it the most are warhammer online, farcry 2 and cod5. Any help would be greatly appreciated!