an extra exhaust fan would just pull more air through, can't really hurt to give it a try, seeing as it's your fun/project box. If you really want to play, look into liquid cooling solutions. You could be the only guy on your block whose computer has a radiator.
I'm probably going to give the exhaust fan a shot. I'm also thinking of replacing the CPU fan because its old and when I got the box, the CPU heatsink/fan was almost completely clogged with dust so I'm sure it worked really
hard for who knows how long.
The only thing I'm not happy about now is the inability to record HD channels from the cable box. I'm restricted to the four or so over-the-air HD channels that I can pickup with the antenna that came with the tuner.