That would depend on the brand of router that you will get, Linksys, 3 Com, Belkin or Net Gear ...

3Com got two antennas and the range of it is really great regardless of being a 'G' series.
Belkin or US Robotics got also a very good range.
Linksys got a 'G' and 'N' series, the 'N' series got a very good range.

The structure of the house would effect the signal strength, you can either get some 'Range Expanders' or get extra antennas which is not applicable to some models.

You can also do a simple trick with some of the desktops and make them act as Routers too, just a simple bridging between your LAN and WAN NIC and you got it ...

 

Do not dwell in the past, do not dream of the future, concentrate the mind on the present moment. (Gautama Buddha)