How do I increase or strengthen my internet connection for my downstairs hardlined desktop computer, when my modem and wireless router are setup upstairs?

I have my TWC modem and Netgear n750 dualband router installed upstairs. I have two macbook pros, an ipad, 3 game consoles using wifi, and cell phones using the wireless connectivity just fine. Some are utilizing the 5ghz bandwidth for heavier media use, and the rest are using the 2.4ghz bandwidth just fine. I have placed the cordless home phones in certain specific spots in the house to minimize interference as much as possible, but I am having some major trouble with my PC. I just built a new computer and I opted to not get a wifi card set in the motherboard since I have a netgear network extender/powerline thingermajig that has a spot for an ethernet cable. I assumed having the extender was going to simulate another direct line to the modem, but it is not working as strongly as I'd like it to work. I don't know what to do now. What is the best next course of action to increase my internet connectivity. It is a nice computer, but without internet it loses its main functioning purpose.

What can I do (or get) to increase the internet signal for my new desktop PC if I want to keep it hardwired, and should I keep it hardwired or just get a wifi card? Also, if that is the best solution, do you have any wifi card suggestions? is it a wifi card or wireless n-adapter to pci that I need?

Any and all help with this matter is much appreciated. I tried to give as much info on the situation as possible, but if you need more let me know.