What do you use the computer for? If it's only for gaming, than the E8400 still holds it's own because of the high clock speed and cache, for most anything else I'd take the Q8300. This is assuming you already have a socket 775 board, if you're building new you can do much better for the money than either of those.
E8400. It's a great all-around proc, especially great for gaming. Despite "only" having 2 cores, it'll still hold it's on in multi-threaded tasks.
Besides, multi-threaded support for applications is still lacking in most cases. There's a handful of them out there, but unless you do a lot of media work, you're better off with the E8400.