With the WRT54G you can try setting up QoS in the Applications & Gaming section. I have mine enabled and set my wired ports to have higher priority than wireless. This may help increase your throughput to those devices.
Also, not sure if your router is susceptible to this, but some don't allow simultaneous connections. So basically when you have multiple machines running on the same router, it essentially switches back and forth between the connections. This can cause what appears to be lag or poor connections.
Hope that helps