Luck of the draw I imagine, its a sl35d if I remember correctly, I had one and it did about 630, and I hope its really good pc100 cause even at 600 mhz it's a 133 fsb.
With a 600 core and an Alpha provided you have ram capable of 133mhz then 600 should be no problem. Unless you have some VERY good pc100 I wouldn't even hope for anything above 600 with that ram.
we got one upto 150mhz bus... but talk about a LOT of work modifying an alpha AND a vantec to fit on a single chip (similar in concept to the old double stacked celeron coolers). Poor chip only lasted a week running like that