Are you opposed to running cables? One option is to have a wired "backbone" that connects more than one wireless access point.
I'm not sure if the belkins are fully compliant or not, but in theory you can have the same SSID and security settings and you'd seamlessly roam between the APs...

If that doesnt sound attractive, you may have some other options...
The cheapest method is to make your own parabolic antenna
http://www.freeantennas.com/projects/template/
with some foil and cardboard you can greatly increase the distance of your AP.
After that try a new external antenna, if the belkin supports it... and place the antenna in an open area like a stairwell...

Finally, you could consider something like an Apple Airport Express that will do wireless extending...it picks up the signal from the belkin and re-boardcasts it... but it comes with some overhead and isn't as elegant as it sounds.... there are tons of other models that will do WDS, but the AP Express is the first that came to mind (and probably the most expensive of them).