PCI is 32 bits @ 33 MHz = 1,056 Megabits per second, raw bandwidth
PCI-E x1 is 2,500 Megabits per second / 10 = 250 MB/second
So, both have enough raw bandwidth to support GbE (Gigabit Ethernet).
The comment above is correct: the old PCI bus is shared
across all devices that use it, whereas PCI-Express does
not suffer from that limitation.
We run Intel's PCI GbE adapters in several workstations,
and they are very fast: you have to factor in the
firmware and hardware efficiency of the adapter itself,
particularly when 2 of the same Intel GbE adapters
are communicating directly with each other.
Cross-vendor GbE tends to slow down, in our experience.
MRFS