3. Upgrade to Wireless N ($50–100 for Router and $30–$70 per Adapter)
If you've ruled out your router's positioning and interference from hindering your signals and you think there's just not enough range, consider upgrading your devices to 802.11n (or Draft 2, as the standard isn't completely finalized).
This involves buying brand new equipment; 802.11b or g isn't upgradable with new firmware. Wireless N (as we'll refer to it) uses multiple-in-multiple-out (MIMO), a smart antenna technology, and other improvements that make for much larger coverage areas and faster connections.
If you do want to make the upgrade, make sure to use the same manufacturer for all the equipment because of interoperability issues with Draft N and proprietary features.
To get full improvement, you also need to use both Wireless N for the router and the wireless adapters.
Remember that faster wireless doesn't mean faster Internet, however. The 4Mbit/s or so Internet connection is a bottleneck to the 20Mbit/s or more (real-world) Wireless G speed (or 100+Mbit/s Wireless N speed).
Faster wireless, though, does mean faster file transfers among the computers on the network and better support for HD streaming and other high utilization applications.
Because of lower prices on Wireless N gear, I recommend upgrading the core equipment instead of just replacing stock antennas with higher gain ones, like you might see discussed elsewhere.
The high gain antennas can run $30 and up. You'll see similar range improvements when spending $50 and up on a new Wireless N router. For more range, upgrade each computer's adapter to Wireless N over time.