Why wouldn't overclocking help if he is crossfires?
@OP: Sandy Bridge is definitely the way to go if you can afford it. I would recommend the i5-2500k. It is very easy to overclock and with decent cooling should reach 4.5ghz without much of a problem. Here is a comparison with the 955; http://www.anandtech.com/bench/Product/288?vs=88
The above ignores overclocking so the difference is actually significantly greater if you were to do so.
Its very easy to overclock.It is even easier to overclock black edition cpus.You have to increase the CPU multiplier.Thats it.Also hundreds of guides are available online for advanced overclocking.
A decent 500-600Mhz OC is easily achievable.And yes you would see difference.