Alexh is spot on. See
here for more details. With the latest mask, 60MHz is fine with only a decent heatsink (watch the height). 80MHz is almost a guarantee with some additional precautions and fussing. Mine ran stable that way (stable as in crucnching RC5 for days without a hitch) for 3 years or so before I got bored and packed it away back in the closet. (I had the 1260@80, Indivision, Melody Pro, IDE-Fix Express, and a PicoPSU all in the desktop case, and it ran like a dream.)
Above 60MHz, I would consider active cooling. At 66MHz, it might seem ok casually poking around in Workbench, but after an hour or so of Quake or RC5 it will get hot.
One last edit - because the bus speed will be divided at anything above 50MHz, esoteric SIMMs are not really that essential. I tried several 60ns SIMMs, at 80MHz they all worked OK.
