while it may seem ok to you, and it is for a short time IMO, for the sake of a larger low rate fan mounted to the front inner case your going to get a far better airflow to the back and also help the cpu fan do its thing too.
personally, id rather try and source the largest copper heatsinks with lots of long fins, and fit these to a main encoder machine (you got an ati X1600 or better for the GPU encoding ?) cpu/northbridge/gpu/etc rather than rely on fast and noisy fans, but it seems really hard to find large cost effective copper heatsinks in your local computer shop, arr well
any req on current silent (or near as possible) PSU's that are cheap and plentiful, to spec too is always a good thing, my current one is on its way out and the 5 V is wondering below 4.5 v at times Arrrrrrr.