Wireless to Basement for XBox Live

I currently live in a 3 floor house, a basement and two top floors. I've been having trouble getting a strong wireless signal for a while now and have tried multiple things.

My current set up is a wireless-n router in the kitchen (1st floor) that services 3 laptops, a PC and a XBox 360 (never all at the same time). The problem is with my 360. It's in the basement, and it lags out or loses signal all the time. It has gotten much better when I upgraded to Wireless-N, however whenever I want to play on XBox Live, no one else in the house can be using their laptop or I will lag out and lose signal in the basement. This is a problem as my family is quite often on their laptops when I'm trying to play XBox. Is there anything I can do? Moving the router isn't a good option I believe, as my DSL company has only installed it so that the router works through only one phone line jack.

Any suggestions? Thanks!