The card in question is an HP NC380T. The spec sheet says its compatible with PCIe-x1 slots, but it doesn't physically fit. Well, it didn't anyway. I've done a bit of surgery, and now the card fits the x1 slot just fine:
|Card with nibbler and kitty. I made that square notch.|
|Comparison with an unmolested card|
I read somewhere that a 1x PCIe 1.0 slot provides up to 250MB/s. These are two-port cards that I'll be linking up at 100Mb/s, so I'm only using 20% of the bus bandwidth. The single lane bus would be a bottleneck if I ran the cards at gigabit speeds, but I expect to be fine at this speed.