Of course other people on the same channels as you will cause issues with your wireless but it can be anything from cordless phones to microwave ovens that causes issues.
Wired machines will always run faster than wireless. You should try to run as much as you can wired to reduce the competition from your own machines.
Replacing the router will make no difference.
Your latest post is a different problem. If your speed varies using a single machine on a wired connection you are having issues between your house and the ISP. When you are using wireless it is more likely you are having interference issues INSIDE your house. I assume if you take your PC that is getting 200k upstairs by the router the speed increases. You can't fix this, wireless tends to get slower the farther away you and and the more stuff you have in between. In addition the further away you the more likely you are to be affected by interference. This is just how wireless works.
All option to fix this require some other hardware to either increase the signal level or to replace the wireless with another form of network. Many people use a cable to extend a AP closer or use things like ethernet over power. There are also systems that will extend the network using the TV coax. The simple option is to run a long ethernet cable if you can get it though the walls.
A duplicate IP will cause you all kinds of problems but the most common is it does not work at all. It is one of those things you should always fix since it is easy.