On the power saving from a new consumer router, hate to tell you, they really aren't anymore efficient (12v/5a=60w) as the rating on my current Netgear unit shows.
The ratings on a unit aren't going to tell you much about the actual power consumption. A consumer router isn't going to pull anywhere near as much power as an old P3 system. If you have a killa-watt or something similar lying around you can test it yourself. I've tested several routers in the past and can't recall the exact wattage, but I know they were all less than 5 watts from the wall. All of my old coppermine based systems pulled anywhere from 20-43 watts at idle depending on the motherboard, add-in cards, hard drives, whether or not it was undervolted, etc...... I don't recall exactly what the lowest wattage was, but I do recall it was in the 20's. These were stripped down systems running as a NAS box with 1 drive for testing purposes.
The only way I'd consider using an old system instead of a consumer grade router was if it offered something the off the shelf router didn't. Until recently I was running Untangle, which did far more than my router, but it requires something beefier than a P3.