Exchange 2007 to 2013 CPUs

I am having an issue with our exchange servers. Currently they are using 8GB RAM and 2x Intel Xeon E5420 cpus at 2.5GHz. Our usage right now is 95% CPU and 80% RAM usage all the time. I am in the process of upgrading our exchange server but I can’t figure out what configuration would be the best hardware wise for Exchange 2013. Any ideas?