PCI 64 bit & 32 Bit V's 33Mhz & 66Mhz
Here's some additional reference info that might help...

When you're talking about a bus, like the PCI bus, the
main performance criteria is BANDWIDTH. Bandwidth 
is a measure of the amount of data that can be moved in
a given peroid of time, usually measured in 
Megabytes per second, or something like that. Bandwidth
is a product of two other factors, bus width, and bus 
speed. Think about a water pipe. The amount of water it
cam move is dependent on (a) how WIDE the pipe is, 
and (b) how FAST water can flow through it. Obviously,
a wider, faster pipe is the best. The PCI bus is the 
same - a wider bus (64 bit) is better than a slimmer
one (32 bit), and a faster bus (66MHz) is better than a 
slower one (33MHz).

As to 64bit/33Mhz versus 32bit/66Mhz, they provide the
EXACT same amount of bandwidth. It makes sense, 
if you think about it. You're cutting the pipe's width
in half (from 64-bit to 32-bit), but you're doubling
the flow 
rate (from 33MHz to 66MHz). The result is the exact
same amount of bandiwdth. For reference, here are 
exact bandwidth figures for the PCI bus:

32bit/33MHz - 133MB/s
32bit/66MHz - 266MB/s
64bit/33MHz - 266MB/s
64bit/66MHz - 532MB/s

  PCI Local Bus

PCI Mechanical Specification

PCI boards have two basic form factors, standard or
long length (312 mm) and short length (174.63
mm). Variable height 
short length (119-167 mm) boards are also
available. Edge connectors are keyed for 3.3V
signaling, 5V signaling, or 
universal signaling. Universal boards are designed to
fit in 3.3V or 5V connectors.  

The 32-Bit, 124 pin PCI connector has 120 signal pins
and 4 key pins. The 32-Bit connector defines the system
signaling 
as 3.3V or 5V. An optional 64-Bit extension is built
into the same connector molding extending the number of
pins to 184.  

A 32-Bit PCI board identifies itself for 32-Bit
transfers when it is installed in a 32-Bit or 64-Bit
connector. A 32-Bit 
PCI board can be installed in either a 32-Bit or 64-Bit connector. 

A 64-Bit PCI board identifies itself for 32-Bit
transfers when it is installed in a 32-Bit connector. A
64-Bit PCI board 
identifies itself for 64-Bit transfers when it is
installed in a 64-Bit connector.  

The signals that enable 64-bit operation are REQ64 and
ACK64. They are Side A Pin-60 and Side B Pin-60 of the 
32-bit connector. 

64-Bit PCI boards do not fit into Ultra 80, Slot 4. 

64-Bit PCI boards do not fit into Slots 9 and 10 on
A20/A25 System Boards 501-5028, 501-2996, and 501-5270.  

PCI Electrical Specification

The PCI specification provides for 3.3V and 5V
signaling. Signaling is determined by the
motherboard. Signaling for a 
3.3V PCI board is at 3.3V. Signaling for a 5V PCI board
is at 5V. Signaling for a universal PCI board is at
3.3V or 5V.  

All PCI connectors require four power rails: +3.3V, +5V, 
+12V, and -12V. The distinction between a 3.3V and 5V PCI
boards is in the signaling protocol, not the connector power rails. 

The maximum power allowed for a PCI board is 25 Watts from 
all four power rails combined.