well, the first thing we have to realize is that being 64 bit or 32bit is only one of the characteristics of a PCI slot. There are a couple of others that are important also and these are signaling voltage levels (5v or 3.3v) and speed (33Mhz or 66Mhz). All of these things are factors but the signaling level is one of the more important issues when considering wether a card will work in a certain slot or not. The other 2 factors are usually less of an issue because the PCI slot will default back to work with the card. That is a 64bit PCI slot will default back to 32bits if the card installed in it is only a 32bit card and a 66Mhz PCI slot will default back to 33Mhz if a card that only supports 33Mhz operation is installed. When it comes to signaling level, however, a slot expects a card to support it's signaling level and it must do so or it either won't work or bad things will happen. To prevent this slots are keyed for either 3.3v or 5v signaling levels by the placement of keyway in the slot and the corresponding notch in the card. Thus if a card doesn't support the signaling level used by that slot you won't be able to insert it in the slot because the keyway won't line up with the notch in the card. A slot will always be keyed for one or the other (3.3v or 5V signaling level) and not both and thus will have the keyway placed in one of the 2 locations. Cards on the other hand can be keyed for either or both and thus either have a notch cut out to fit the 5v keyway, the 3.3v keyway, or they can have both notches cut out so as to fit into either a 3.3v or 5v slot. A card that has both notches cut out is a universial card and it will work in either a 3.3v or 5v slot.
So, the answer to your question comes down to a couple more questions that you must answer and those are whether or not your 64bit slots are 3.3v slots or 5v slots and whether your 32bit card is a 5v only, 3.3v only or universially keyed card. Most of the newer 64bit slots like the ones you see on Xeon and AMD 760MPX boards are 66Mhz slots and a requirement of the PCI spec is that for 66Mhz operation 3.3v signaling levels must be used. That means those slots are 3.3v slots. So, in order to use a 32bit card in those slots it must support 3.3v signaling levels and be keyed appropriately either for 3.3v or universial. Many older 64bit slots, like the ones on old Intel boards and AMD760 boards, are 5v slots that only support 33Mhz operation. Those slots will accept a 5v only card or a universial card.
A couple of things you might want to keep in mind however when using your 64bit slots is that if a 33Mhz card is installed in a slot that is 66Mhz capable not only does that slot itself default back to 33Mhz but also any other PCI slot that is on that same PCI bus or device that's on that same PCI bus must default back to 33Mhz operation. This includes onboard or system devices that may be connected to that same PCI bus. That means that installing a 33Mhz card in one of your PCI slots may slow down any other cards you install in the other 66Mhz slots if those slots are on the same PCI bus. I believe in the 760 MPX chipset all of the 64bit slots are on the same bus. Also to make matters worse I believe the southbridge used with the 760MPX connects to the northbridge through the 64bit 66Mhz PCI bus. So, if you install a 33Mhz PCI card in one of those slots it may slow down this connection and may in some situations impact system performance.
Another thing to keep in mind is that while 3.3v signaling level is required for 66Mhz operation that doesn't mean that a card that supports 3.3v signaling level and is keyed as such or is keyed universially will support 66Mhz operation. Many cards that support 3.3v operation will only support 33Mhz operation. This isn't a problem as the card will still work in a 3.3v 66Mhz slot. It's just that the slot will default down to 33Mhz operation. Clock speed is determined by one of the lines on the PCI bus as described here.
http://www.techfest.com/hardware/bus/pci.htm#4.8
M66EN
66MHZ Enable is left "open" or disconnected on add-in boards that support operation with a 66 MHz CLK, and grounded on add-in boards that support operation with only a 33 MHz CLK. 66 MHz systems place a pull-up resistor on this signal to detect if the add-in board is 66 MHz capable. If the signal is high, a CLK with a maximum frequency of 66 MHz is supplied. If it is low, a CLK with a maximum frequency of 33 MHz is supplied. 33 MHz systems attach this signal to ground. 66 MHz operation will take place only if both the system and the add-in board support it.
Here are a couple of good links for more information about PCI slots, cards, keying, and which cards will work in which slots.
http://www.techfest.com/hardware/bus/pci.htm
http://sunsolve.sun.com/handbook_pub/General/PCI_Info.html