Glad to know that you want to understand tomato source code base. However it will be very difficult to describe it in a thread. I will try to make a blog quickly on that. In the mean time if you have any specific query then please tell.

In general you can find network driver code under linux/driver/net path. However here Ethernet and wireless driver code are not put into that directories. You will find the source code of Ethernet driver in src/et/sys directory. Unfortunately there is no source code for wireless driver of WGR614L. You can only find the object code in src/wl/linux directory.

you can run a image from RAM also.... for that you need to set one nvram variables.
1. Get into the CFE prompt. Execute the following commands -
'nvram set boot_wait=on'
'nvram commit'
2. reboot the router. Now the router will search for vmlinuz from 192.168.1.2.
3. So connect a host in the router's LAN side and set it's IP to 192.168.1.2
3. Run tftp server in this machine and place your vmlinuz in the /tftpboot folder.

Now the router will boot from vmlinuz file placed in 192.168.1.2

Non-volatile random access memory (NVRAM) is the general name used to describe any type of random access memory which does not lose its information when power is turned off.
To boot the image from flash you must set the host ip to 192.168.1.2. Because when the boot_wait os set to on, by default CFE searchs for vmlinuz from 192.168.1.2 and if it does not find anything then boots the flash image. So you need to assign this ip to the board if wish to boot from RAM.