Thanks for all the replies. That is a good idea to try both NICs at the same time; I will try it soon.
The bottleneck is not the CPU... when measuring performance, the CPU is at 17% load while the bandwidth is topping out at 3800KByte/sec.
I have the onboard 10/100 disabled right now. I enabled it only temporarily to see if the speed would exceed 3.8MB/s (which it did not).
Regarding the OS's: I have tried both 2000 Pro and XP Pro both with the same exact result.
What is this module faul you speak of? Any more information would help. Thanks!