I currently use the DD-WRT Linux-derived operating system on my WRT54G, which allows me to use the SD card hack and a few other features not available in the stock Linksys firmware.
I definitely recommend looking into a replacement firmware for your WRT54G. You’ll find that you can do much more with custom firmware. DD-WRT is not your only option, there are many others such as OpenWRT. Some are optimized for things such as VOIP, while others for gaming and such. A little Google searching will help you find the one best for your needs.
Also, be sure to check out the DD-WRT wiki, which is a really excellent source of information. The help files in the firmware itself are also surprisingly detailed.