Changing Intel Core 2 Duo E8400 to Intel Core Quad..But which one?

[jaguarskx]

Overall the Q9400 is better.

http://www.anandtech.com/bench/Product/53?vs=76

 

[amuffin]

Q9xxx are the top of the line core 2 quads. My dad is running a Q9650 and it is serving him well I would stay away from the 8XXX because they are missing some part of their cache..