Zorro ethernet is probably your best option. Good performance, and cards run in the $100 range. PCI Ethernet will be way cheaper, but it'll cost up to twice as much as a Zorro card to get a PCI busboard. (Depending on your future expansion plans, this may be something to look into though.)
There are no clockport Ethernet interfaces, so that's not an option. Several have been designed and there might be some prototypes, but none have (yet) made it to mass production. that I'm aware of. A USB->Ethernet adaptor might be a possibility in the near(er) future once drivers are written.
The Null modem will be super cheap, but the bad speed will probably drive you insane.
