The WRT54G is the standard 54Mbps version of the router. The WRT54GS is the version with SpeedBoost incorporated and, when used in conjunction with a compatible Linksys Wireless Network Adapter with SpeedBoost should connect at 125Mbps. I have this kit (although I no longer use the Linksys router) and have yet to be convinced that there is any speed increase at all.
I actually contacted Linksys and asked if, in the adapter's Properties, it should show as connected at 125Mbps rather than 54Mbps. They couldn't (or wouldn't) give me a definitive answer.
I mainly use a D-Link DI-524, or Netgear WGR614, rather than the Linksys which had a habit of losing connections to networked computers about twice a day. I think the WRT54G was a more reliable product than the SpeedBoost WRT54GS version.