AMD X2 4000+ = 2 cores, each of 2.0 ghz.
INtel C2D 2.4GHz = 2 cores, each of 2.4 GHz
INTel Dual core = dont even look at them, they are not worth buying if youre looking for performance.
For a bit of explanation, look here:
*www.thinkdigit.com/forum/showpost.php?p=637846&postcount=6