How can I fix my Mac-Router-Mac network?
Looks closer to 802.11b than 802.11g, the speed advertised for wireless networks is confusingly not equivalent to an Ethernet cable of equivalent speed.

This guide will be informative for you:

http://www.smallnetbuilder.com/wireless/wireless-basics/31083-smallnetbuilders-wireless-faq-the-essentials

Basically buy an Apple Airport Extreme to replace your current system, and try to use the 5Ghz frequencies instead of 2.4Ghz as you are probably suffering from a lot of interference.

Example setting up with existing router: