Yes. Systems with PCI-X slots are 64-bit as well as backwards compatible for 32-bit cards.
However, one must be aware of the limitations when running simultaneous PCI and PCI-X cards, or PCI-X cards of differing speeds. The PCI and PCI-X standards are based on a shared bus architecture. This means that when multiple PCI or PCI-X cards are installed in the system on the same bus, they will automatically run at the speed of the slowest card.
For example, in a system with two PCI-X slots, if both a PCI-X 64-bit / 100MHz card and PCI 32-bit / 33MHz card were installed, both cards would run at 33MHz, therefore affecting the performance of the faster card.