Best deal an airfare from UK to Chicago USA?

Hi, i'm looking for best deal on flite from UK to US, round trip. Would prefer to depart from manchester uk and end in chicago or NYC but could depart from london also. I have done some research but just started and any tips would be helpful. What might we expect to spend on round trip flite early august to mid august, summer 2011. What's the best online deal site to check for good deals. Thanks for the time..