New GPU will help something like HD7850 but to get all out of it the CPU needs overclock.
to get all out of the HD7850 I need to overclock? or to get all out of the system in general? Please explain. I'm not opposed to overclocking, I just have a very limited amount of experience and understanding of the process.
Your CPU will bottleneck slightly a HD7850, if you overclock the CPU to about 3.5-3.6GHz that will minimize that bottleneck. There are plenty of guides on how to overclock on the web and even in the overclocking section here.