The best AT motherboard was probably the Asus P2B-B, btw. It supported bus speeds up to 150MHz for overclocking the PIII. You could add a PIII 1000EB and push it up from their, mine handles the 150MHz FSB quite nicely. If you look up the info on this board, you'll notice it can use either AT or ATX power supplies. I would recommend ATX power, the shells of AT and ATX power supplies are the same.
<font color=blue>At least half of all problems are caused by an insufficient power supply!</font color=blue>