I hope you made sure your board supports that 6400 before you ordered it
. Anyway while the 6400 is certainly a significant improvement over the 5000, I think your bigger issue is your GPU. Even with the stock cooler your CPU shouldn't have too much of a problem passing 3GHz, assuming it has the G2 stepping anyway. Overclocking the 5000 means stepping up the 6400 will be a smaller gap, so if you're not afraid to overclock you would be better served by sending back that 6400 and investing in a better GPU like a Radeon 4870 (they are just so cheap now ^_^).