Well, you guys managed to get an bottom line AMD processor that is far removed from an I7 that you have. Depending on the type of the game (this one obviously relays on processor ) heavily, hence the poor performance. Just MHz is not the whole thing there, other things to consider. My 965BE @4.2 GHz copes just fine with the same game even coupled with a single HD3850 Top GPU. If you feel tha CPU is bottlenecking you do not have to go whole tilt to 8350, a 6300 series will do just fine for all the games.
Before changing anything you should check if there's something else that's taking some clocks from CPU or overloading the memory. Speaking of memory, how much memory is on that system, less than 1 GB per core can severely hinder system performance.
Also make sure that computer is running on all 4 cores, it is possible that it's running on less cores if OS was not installed anew.