Hi, before you start, I have been an AMD user all my life BUT I am now thinking of upgrading to Intel. I would not recommend AMD for new gaming PC.
If you are updating your CPU I doubt if the Phenom is compatible with your M/B. So the 6400 may be your only option (check out your M/B details first)
AMD 6400 specs
http://products.amd.com/en-us/Deskto...000&f10=False&
Intel 8400
http://www.intel.com/en_AP/Consumer/...ap+c2d_desktop (click on processor details)
The Intel has a 6meg L2 cache and the AMD 6400 has only 1meg L2 Cache.
My PC specs are AMD 6000, 2 gig 800ram, 1 gig GT 800 over clocked, 750w PSU, Gigabyte MB, Vista Ulitame.
Crysis runs at 40-42 fps in only medium settings and I know my system is bottle necked because of the AMD6000.
Just thought I would warn you first.