What changes do I need to make to my wireless network to avoid these 25s ping times?

What changes do I need to make to my wireless network to avoid these 25s ping times?

My house is kind of long-ish, and I need wireless coverage across the whole house. I have 4 roommates, and our rooms are all at opposite ends of the house. The cable modem can't be moved. Running ethernet cables isn't really an option, but many of the devices on the network sit right next to each other. I'm quite willing to throw both money and time at the problem.