Xbox 360 wireless network adapter problems?

I just bought the wireless adapter for the first time. I have a wireless internet and its the linksys one. But everytime I play a game like Modern Warfare Modern Warfare 2 and World at War, I always lagg and have 2–3 bars of connection. I mostly have 4 bars, but i mostly have 2 or 3 bars. I have the home wireless connection one, and I can get good connection for my laptop upstairs and downstairs. But when i logg in to Xbox Live, it laggs.
Please Help.. Thanks