The difference is very much physical, and 64-bit 33 MHz and 66 MHz PCI slots are not new. They are, however, not usually found in consumer boards, which is probably why you're not familiar with them, ...

There are two flavors of 64-bit slots: 33 MHz and 66 MHz. Most of your average everyday 32-bit cards will work fine in a 33 MHz 64-bit slot. They won't work in a 64-bit 66 MHz slot, though. The 66 MHz ...