fwiw,
the extra notch in the pci card means that it supports 3.3v signalling as well.
in order for a pci slot to support 66mhz, it must use 3.3v signalling. most pci slots nowdays use 5v signalling. if you look at a regular pci slot, it doesn't have the extra notch at pins 12,13 and 3.3v only cards won't fit it it.
the newer promise cards are universal cards and will support 66mhz, 3.3v if the motherboard supports it. if not, it will run at 33mhz, 5v. chrisbored is right, the only mb's you see with 66mhz slots are server level and have 64bit slots. note that there is no reason a 32 bit card can't run at 66mhz, if it's designed that way, like the promise, and you put it in a 66mhz capable slot. if you put your promise into a 64bit slot in the tyan, say, it will run at 66mhz, 32 bit.
there are no 66mhz, 32bit only slots on any current motherboard, but i'm sure we'll see it soon.
sorry, this isn't the most succint explanation. i made a similar post in the motherboard section and got the feeling nobody understood.
p.s. having the extra slot is no guarantee of 66mhz operation, only that it supports 3.3v. i doubt the soundcard supports 66mhz, but i don't know this for a fact.