Linksys wifi router problems?

i have a linksys wifi router model wrt 110 and i need to increase the range on it. the router works fine other than the range, connecting to it thru my xbox from upstairs makes my live connection laggy annd it drops out all the time. i was wondering if i could buy an external antenna for the router becuase the model has internal antennas and broadcasts the signal in a 360 degree range, its not directional. i put a little tinfoil makeshift dish behind it to try and boost the signal but it doesnt do much if anything. moving its not an option, please help me out. thanks.