Poor connectivity with router

Hello,
I have just had my computer reformatted and reset up. I set my router up in my den on the 1st floor and my reformatted computer for my boys in the basement. I am averaging about 54% connectivity and can no longer establish an internet connection. When I first started the computer, it worked but was not secure. Once I went to the LinkSys web site and added only the MAC addresses of my laptops, it no longer would connect. Granted the reception rating was between 52%-58%.
Any help would be appreciated. if I need to purchase a more powerful router I will do so. Thanks again!!!