Like Biggs was saying, it's more often going to be a specific PCI device not working correctly when it's out of spec, but that's not too badly out. I would not try to take it any higher than 39-40Mhz though cause then you will have problems with the AGP as well when you're talking 78-80Mhz for the AGP which is normal at 66Mhz (37Mhz PCI is only 74Mhz for the AGP so it should work ok but you will have to check if your video or anything else freezes when using the device).