Okay, so a 5.4Ghz phone will not interefere with a Wireless G router. A 2.4Ghz phone will because it runs at the same frequency (you'll typically get some background noise and possible phone echo) and sometimes if it's real close to the router it cuts off, but you should have no problems with a 5.4Ghz.
Now, most Wireless G routers are built the same with some little variation. A Wireless G router can normally handle up to 54Mbps, so you should have no problem with handling 5Mbps. However, the speed of the internet can vary depending on how many computers you have linked to the internet at any given point and time (I've found 2 to be okay on a Wireless G, but once you start 3 and more it tends to slow down a bit).
As for distance a Wireless G router usually can handle anywhere from 100ft-150ft depending on the router and that is total distance away. Of course the closer you are the stronger the signal. The one at my friends house is 50ft from using it out by the pool to the router in an upstairs bedroom and he gets about half a signal strength outside if you want a comparison. If you think that will be a problem, just upgrade to a Wireless N, usually just a little bit more.
Hope I was not too late to help. Had to write this twice because work computer froze. Have a great day!