Old vs New: Core 2 Duo E7400/8400 Vs Core i3 530/Pentium P6950
 

src: http://www.inpai.com.cn/doc/hard/115589.htm

in my humble opinion, if you have a Core 2 Duo system clocked at least 2.8Ghz or higher, your only upgrade path should be Core i7 or AMD X4.

a fairly impressive increase clock for clock