I would suggest you split your users in 2 and have 2 different networks and not consider seamlessly moving from from one AP to the other. You can also split your "High usage" staff in half for each AP and reduce the possibility of overloading one AP.
My one experience with a "Range Extender" is that I did not not get the increase in range I expected. It came with a fixed antenna and since it was on loan I couldn't dismantle it and replace it with a higher gain one. There are a number of AP's on the market which will work in WDS mode:- Wireless Distribution System - Wikipedia, the free encyclopedia
Some of these have a WDS+AP option which is very similar to a repeater. I will add one disadvantage to the list in that for a lot of these AP's I have only been able to get WDS to work properly if all the WDS capable AP's have been from the same manufacturer. I have not tried any wireless routers using 3rd party firmware for this mode.
If you decide to use some form of "Repeater" with a high gain "Omni" antenna then bear in mind that the height of the antenna and the Vertical beamwidth, normally quoted in the antenna spec, could mean that users close to the "Repeater" may not get a good connection. Omnidirectional Antenna Beamwidth Analysis
When I was asking about the version of the router I was not referring to the firmware, the WRT54G has at least 8 versions and then there are GL's,GR's,GS's....
There are different versions of 3rd party firmware and you will need to check your version of WRT54G to see if it will work with one of the versions of firmware. Some versions of WRT54G only had a small amount of RAM and the full version of firmware could not be loaded.
Free firmware:-
Tomato Firmware | polarcloud.com
Main Page - DD-WRT Wiki
As long as you follow the installation instructions precisely you should be able to try these and if you don't think they are of use you can re-install the Linksys firmware. DD-WRT has been around for quite a while and as long as you use a stable, non-beta, version you should be alright.