Both are socket AM2 so it should be compatible. Im not sure if theres more to upgrading a cpu than making sure the sockets are the same, e.g BIOS update ,etc?
Yes, it will be compatible because it's using AM2 socket. But instead of getting a 6400+ you might want to get a 6000+ instead, because it's cheaper. They are both 90nm, same cache, and you can easily overclock it to 6400+ speeds.