Atmel MEGA128A GSM Project add IP interface

I have a project already done my a good coder on here that reads data from an RS485 device, displays a status on an LCD and uses a GSM modem to connect to a webserver and upload the data to that server all over GSM. Also the webserver is used to configure the MCU to what devices to scan and report. There is a small serial program that I use to load APN settings and device ID into the MCU locally, then everything else is via the webserver. Now I need to add an ethernet interface, so I can choose to use either the GSM modem, OR a standard IP connection to the internet. I already have the circuit working fro the GSM only version, but will need help with adding ethernet to the original design. I am able to layout the new PCB myself no problems. I have seen the Microchip ENC28J60 Ethernet Controller Chip has an SPI interface and minimal external components. Just to be clear the project is working already with a GSM modem via a serial interface, I only want the Ethernet interface added, I do not need the whole project designed from scratch. The MCU is a Atmega128A