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[carpetsmoker]

no, if you have a 100mbit card then this is 100mbit full-duplex.
If you run half-duplex then you have 50mbit.

Note that this are theoretical speeds, You will never achieve 100mbit, this is because of extra meta-data (mainly headers) added by various protocols in the network layers.

Also note that half-duplex will always be less than half the speed of full duplex, this is because your NIC's will have to wait until the line is clear (turnaround time).

I don't think it would. Besides, PCI bus would be a major bottleneck for that anyway.[/b]

PCI 33MHz has a theoretical speed if 133MB/s per second, which is way faster than 100Mbit, and even 1Gigabit.
If I remember correctly most computers nowadays have either PCI2 (66MHz) or PCI2.3 (100MHz), which are even faster.

doesn't that mean it transfers twice the non-duplex capacity[/b]

Infinitely faster, since non-duplex is 0mbit/sec

You can either have:
simplex - one way traffic, for example your TV
half-duplex - two-way traffic, one machine sends, other receives, can't send and receive at the same time.
full duplex - both machines can send and receive at the same time.