Much complication and hackery here. You only get one address from the cable company. Run a ethernet cable to the garage from one of the 2500 router ports and put one of the other routers on the end of the cable. You should not have to do any configuration other than normal security (WPA pre-shared key). Consumer wireless gets maybe 100~150' tops, depending on the environment. G band will go farther than A band and if you have a chain link fence between you and the garage, get the router up way high to try and shoot over the fence. Chain link is great for killing 2.4 Ghz wireless.
This is if he really wants wireless in both locations.
Essentially what you are suggesting is that he use one of the router ports, run a line to the garage, connect another router (but ensure that DHCP is disabled, the connection is plugged into the LAN port and not the WAN/Internet port), set WPA2 security (WPA is easily cracked along with WEP).
There are instances where the wireless signals will battle each other if they use the same SSID and they are within close enough signal range with each other. Sometimes you will get a device that keeps switching between the two signals. That is why for corporate or large scale deployments, wireless access point signals are usually attenuated in order to provide good coverage with minimal overlap to prevent wireless connection issues.
But, by using the powerline networking, he doesn't need to run any ethernet to the garage as he can piggyback on the home's existing wiring. Then at the end, he can throw a switch (for hardwired connections) or another wireless router (for wireless and hardwired) in the method described above.
In case anyone's interested in a couple of fine networking programs... I'll toss out a couple I use:
- The Dude; it's a network mapping program
- TeamViewer; it's a program that lets you
- Rremotely control/contact computers
- Print
- File Transfer
It's a very useful program.
It ain't much, but it's working fine for me....
I personally like NMAP for discovery and LogMeIn for remote maintenance, but great recommendations!