My 2-pence worth:
Tom's hardware site suggest the optimum idle to be around 50C. Certainly my machine (P4, 1.8) standard fan installed reports a whisker under that from the BIOS (the system in general is reported as running at 45C or thereabouts).
I don't think it's a good idea to remove blanking plates as it tends to destabilise the airflow. You might want to get an exhaust fan to force the hot air out of the box. I found it to be very effective.
amowf