You might want to look into the 3C509-TP. They are EASILY available all over the place. It's a 16-bit card but it works in an 8-bit slot too, but needs a hacked packet driver to work on an 8088. (I can provide it if you get one, or if you just use a V20 I think the regular driver works)
I don't know the exact amount of buffer RAM, but it's a lot newer than most of the cards you'd find in an 8088. I suspect it's got a pretty large amount. Probably 32 KB.
Just to clarify;
Only the 3C509B works in an 8 bit slot. The older non-B variants need a 16 bit slot.
The B variants have a 32 KB buffer, the non-B only 4 KB.
They both do only port I/O so a NEC V20 processor would be preferable BUT nestor's "hacked" driver will not benefit from a V20 processor and the original driver will not work at all with anything less than a 286.
You would also need an AT machine to configure the card (change the IRQ) before using it in an XT.