For those Linksys WRT54G series owners upgrading their wireless networks to newer devices that don't support OpenWrt, I recommend using your old WRT54G as a Quality of Service (QoS) device. Placing it at the end of the topology, right before your WAN (modem), you can utilize the wonderful QoS features in X-Wrt White Russian to prioritize network traffic going over the WAN. You can also use the QoS of Kamikaze, but if White Russian does the job, it is the more mature platform. Tomato is another option, though I can only speak for the performance of QoS in OpenWrt. DD-WRT's QoS used to be completely broken, though that may have been fixed now.
QoS can greatly improve network responsiveness when you're reaching the cap on your WAN. In addition to improving the standard browsing experience while downloading in the background, it will also improve your gaming, VOIP, ping, and TCP connection initiation latency.