A PPGA 533 with slotket is probably as good as you'll do. You may need to flash your bios to the latest. My Asus P2L97 lx board has 66/75/83 fsb , but the 83 really screws things up, so I've got my cel 500 running at 563 on the 75mhz fsb.
I've read on the Asus forums,though, that the latest beta bios will support coppermines with a slotket that can appropriately lower the voltage. They would be running slower than rated,though, on a 75mhz bus.
What's the make of your board?