They have to be designed that way, but some work...
Just built a PC that uses a Highpoint 1820A PCI-X card... I had it working
fine in a normal PCI slot on my P4C800E-Dlx before building the machine it
was intended for.
Some 64-bit PCI cards are backward compatible with 32-bit PCI slot. The
cards such as Adaptec 29160 or 29320 (64-bit PCI card) will work with
existing 32-bit PCI slot.