I hope this it's not a late answer to your message, I would recommend WRT54G for my own experience, it's a wonderful router/AP that gives you the opportunity to install new firmwares which gives you extra power for your signal strenght.
I live in a house with concrete walls, very thick indeed, the worst enemy for wifi signal, and I can tell you that this little bastard (WRT54G) gives me the chance to take the signal upstairs (kind of weak but reliable) going throught appoximately 4 thick walls and I haven't touched antennas yet, if I do that I could get more signal upstairs maybe.
The difference between WRT54G and WRT54GS is a 'protocol' which will guive you more speed in your internal transmission (home's internal LAN) WETHER you have a Linksys compatible GS antenna. BUT, WRT54GS version is a little bit more difficult to manipulate with 3rd party firmwares than WRT54G, not only because some firmwares don't work on WRT54G, but Linksys' firmwares aren't the same and a 'bricked' router/AP is harder to recuperate on a 54GS than a 54G (I've had some experience on both).
Conclussion: I recommend gettin' into WRT54G instead of WRT54GS or any other.