Upgrade e6400

i would get this one:

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

but, that was just to answer your question...buying that is like wasting 200$. at that point in your computer, it would be much wiser to get an i5 build, or am3 build. throwing any money into an OLD 775 socket build is just like life support...its going to run decent for a bit, but you are just postponing the inevitable.