I'd also take out the original AT PSU and replace it with another one, as there are reports floating around that it may cause damage to the P8/P9 motherboard connector in terms of burning out the plastic around it somehow. Not really sure why this happens, but if you do a search on A.org forums you are very likely to find some more info on this topic.
Get a quality/powerful ATX psu and use an ATX-to-AT-converter with a switch for the best results.
