Most games do not fully use four cores.But since the quad is cheaper, it would be better to get it because you will notice some improvements in real world scenarios.
You can compare them here: http://www.anandtech.com/bench/Product/55?vs=89
In games which are optimized for two cores, the E8500 would thump the Q8400. But on games which require more cores than 2, the Q8400 would be a nice one. Q8400 was decent quad core. Intel's Q9xxx quads were more powerful and a lot better.
E8500 is definitely a top line dual core. Only thing i heard that its a little unstable when you overclock, that's why E8400 is better and sold more.
I sorta have to pick from this list. Didn't see the Q8400 on the list and i heard i might need a bios update for it to work and that's a bit of trouble. If it was 100% compatible then i would go for the Q8400 since it is cheaper and the 4 cores would be pretty nice.
Thanks for the chart, gave me a little insight.