Howdy Hayne
Any PCI slot network interface card (NIC) would do fine, provided you have a spare PCI slot. You could get one that supports WOL if you want to use that feature. A basic PCI NIC will only cost about US$5 I think.
Once you have a NIC installed, go to
www.practicallynetworked.com/sharing for ideas about how to share the internet connection.
Or come back here with any questions.
Cheers
Mick
BTW, this should be in
Networking and Internet forum. I'll ask a mod to move it for you.