The 040 on my BlizzPPC is passively cooled at 25MHz. Seemingly, this is enough. However, all 3 of the Apollo 040 boards seem to run hotter and are actively cooled. I replaced the fan on one with a low profile intel fan+heatsink that only adds about 1cm in height and is relatively quiet:
On another card (for a tower machine) I started on a slightly more ambitious custom cooler, derived from a P4 laptop heatpipe based cooler (not actually fitted in the image below, just positioned for a basic measurement):
