The newer tech would always give you some advantage. Plus Q8400 is pretty average quad. The Q9xxx series were better.
In games as zipzoom mentioned, which are not core dependent the i3 would rule. But in multi tasking apps your Q8400 would dominate. So its basically down to what you use your PC for.
Take a look at this mixed bag:http://www.anandtech.com/bench/Product/118?vs=89