Your gateway, is your suppose to be your router, and it is most likely 100MB, you dont need it more for the reason of internet speed limitation on 20MB, and avarage backbone speed no more 700KB.
You can do 1GB on LAN, by connecting everything into 1GB switch, and looks like that what you have.
So between local PC, you will have , but real speed you will , when you transfer files between PC, and due to PC's limitation you will have no more than 80MB on transfer speed.
Your internet speed testing website will show you 20MB, but real backbone internet speed usually no more 700Kb at average- you can see that maximum when you downloading files from somewhere, WebPages, yes - can be opened at higher speed depending on hoster, but different in 0.5 sec on opening webpage is not crucial for most users.
In other words having 5mb download/5mb upload is practically the same as 20mb download/10MB upload, due to "internet backbone speed limitation" for residential or small business.
For corporations with 100-1000 people accessing internet, it will be beneficiary have 20MB then 5MB.
Most important, it have LAN perform at maximum, to troubleshoot bottleneck, you can use
http://avcontrol.tv/pics/PCperformanceTools/Summarize saying 1GB is good to have, but technology is moving ahead, and soon CAT6 will be replaced by fiber, currently 1GB is reasonable to use mostly interconnect high traffic links. Once LAN will be fiber base, and all NICs will support fiber(as well PC itself will support fiber transfer, which are limited now by HD Read/write speed mostly ), then you can have above 70MB transfer speed.
