Parallax just came out with an Ethernet Aux board, and the company that make 'em for Parallax (links on their site) also has some Ethernet build-up boards (just the ENET chip and filter).
It's not ISA though. I think you use ~4 pins on the PIC to control 'em ...plus another eight for the raw data in parallel if you want to send it off-chip to another chip or system (or you could pass it serially ... depending on the actual data rate ... the PICs can do 230Kbaud on RS232, or something like 400K on SPI or I2C ... I don't remember.
The CCS compiler has Ethernet code included for most of the TCP/IP functions and protocols.
Digi-Key, Mouser, and other parts places probably have discreet Ethernet chips and filters. 100 meg is a little ambitious for a PIC though. Their hottest chips still only do 40Mhz.
You might want to look at Zilog, they have Z80 Acclaim! chips (with ethernet) up to 50Mhz. Their Z80 Encore! might be able to do it as well, but I haven't seen it mentioned in the same breath as Ethernet ... so maybe not.
There is a company that makes a "Siteplayer" line that is an embedded Web Server. Their Demo board (ethernet and SitePlayer) is ~US$99.00. I've got one, it's pretty slick. You can use it as a one port term server, or feed it HTML that can read or control several digital outputs (like, have a web page that'll control your thermostat / furnace).
Good Luck.
Scott