These cards have a 45MHz oscillator that gives a 180MHz machine through a 4x multiplier, so the bus is running at 45MHz, and that leaves a large area to work with since bus maxes are around 55-60MHz these days. I have not yet clock chipped a 180MHz card, but I would expect them to clock chip reasonably well; maybe 200MHz. So replacing the 45MHz oscillator with a 50MHz oscillator gives you a 200MHz card. Mod 1 and Mod 2 are the best ways to clock chip these machines since the initial oscillators are the metal can type.
At least one person has clock chipped one of these to 200MHz; that was as fast as he tried and it has been stable for over a month. And I've heard from another person who has clock chipped his to 192MHz without any problems.
And another person has swapped the resistors with the PPC604e Bus:Processor Modification (On Takashi Imai's site) and has his card running at 225MHz without a hitch.