You could try putting a 3rd party firmware on your existing wrt54g. Almost all versions of the linksys wrtg4g can be re-flashed to a better firmware. I put DD-WRT firmware on my ol linksys to turn it into a wireless receiver/repeater and it doubled the range of the signal compared to the original firmware. I have a signal at 800m line of sight. Here's a link
to check your router's version and compatibility with DD-WRT. If you really love your WRT54G, don't do this option, survival is not guaranteed. It also helps to have a little computer knowledge but the process is basic and very well detailed.
One more thing, I have an archos gen7 series 5. In order to connect to the internet, I have to be close to the router (<25m), but once connected, the internet works even at a distance where I previously couldnt connect. Also, there is a certain way to hold mine (or not to hold) in order to achieve better wifi. Check you owners manual in your settings. And, if you are tired of getting nickeled and dimed by Archos for every plugin, check out forums.archosfans.com and search "guy fawke". Good day, sir.