How To Expand Comcast SMCD3GNV Range

I'm currently using my Acer Aspire x1700 desktop computer running Windows 8. It's connected to my router via Ethernet and gets about 25mb/sec. The problem is, my router can only broadcast the signal to my two front rooms and all of the rooms of the level it's on (Living Room, Sun Room, & Kitchen). All of my devices get full signal in these rooms. The holidays are coming up, so people are going to start bringing cellphones, tablets, gaming devices, etc into my house. My room is about 24 ft away from it (My 2nd floor is 8.5 ft above the 1st). In my room, my laptop, a Dell Inspriron running Windows 7, got about 13mb/sec. But as soon as I walk into the next room, which is about 8 ft away, the signal becomes very spotty on my iPod touch/iPad. My laptop can pick up about 2 bars, and so can my Grandma's HP. If I move another 2 feet into my mom's room my iPod Touch/iPad gets one bar, my laptop still gets two, but it's very slow and spotty. My iPod touch/iPad gets a kind of weak signal in my room.