I have been using a S508-IW with IWill KA266 and Thunderbird C 1GHz for over a month now. Seems to work fine.
The P4 power supply is compatable with ATX motherboards, you just don't use the extra power lead. It has a good airflow with the extra fan and good sized vents on the front and some on the left side. Have been running system temps of about 39 C, CPU 41 C with Cooler Master DP5-6H51 (AMD recommended).
One thing I don't like...no button on the bezel for the reset. There is a reset button under the bezel, but you can't get to it. On the plus side it is a good looking case.