Using a repeater to extend your WLAN range can be a good way to increase coverage, yet it can also be inhibited by the fact that they reduce throughput on the WLAN.
A repeater must receive and retransmit each frame on the same RF channel, which effectively doubles the number of frames that are sent. This problem compounds when using multiple repeaters because each repeater will duplicate the number of frames sent.
Let me ask this first, why cant you set up the WRT54G to connect to the internet using Cat5 wire and recieve full coverage throughout the house with the extended range activated within its configuration?
Unless your house is absolutely massive, these routers used as Access points have a great range usually. (Your basement must be a black hole...lol)
Having said that, The configuration of a repeater is relatively straight forward.
After switching the WRT54G access point to repeater mode, you set the service set identifier (SSID) (define) of the repeater to match the SSID of the specific (root) access points (WRT300N) that the repeater will associate with.
Most repeaters will, similar to wireless network cards, automatically associate with the access point with the strongest signal. However, you can designate specific MAC addresses of the preferred and secondary access points as an option.
If the repeater cannot connect with the preferred access point, it will try to associate with the next one, and so on.
See how you go!