Xappe was already running Fedora 24, so I didn't really want to reinstell Fedora. Plus, there wasn't a free PCI slot.
But one of the slots was occupied by a sata port card that had only one sata drive attached. So I removed that card and put in a random gigabit ethernet card. Random meaning it was a brand I'd vaguely heard of, but it wasn't an Intel card. That left me with an unattached drive - no problem, I put on a sata-to-pata converter thing, and attached it to a pata connector. I wondered which of those two wouldn't work, the drive or the gigabit?
I closed up the box and started up Fedora. It recognised the drive without blinking, I didn't need to do anything at all. And it recognised the gigabit ethernet card, giving it a different name from the onboard FE (fast ethernet, meaning 100 megabits). But it decided to use DHCP to control it, which isn't too surprising, because I hadn't told it anything about addressing.
To change that, I tried to use system-config-network. But that wasn't on the computer - I should have thought of that before I started, and installed it using "yum install system-config-network.". So that meant I had to change the connection from dhcp to static by editing the config file. Fortunately, I know how to do that, and on reboot, I had a gigabit ethernet.
There's a thing called ethtool that can confirm that for you with "ethtool eth0" or whatever the name of the connection is - you can find out using "route -n".
So Xappe now has a gigabit ethernet, and I'm right now putting that to use by copying a terabyte to it, because one of the drives turned out to be failing, so I'm recopying the data on it, from a backup which is also on gigabit ethernet. Sweet!