Wireless Adapter or Aftermarket Antenna ?

Hello, I need help, as I am not a Wi-Fi networking guru.

Well, the thing is I purchased a Wireless Router (Asus WL-520GC) to go with my Toshiba A300 notebook.
My problem is that the router is situated one flood beneath me, and the concrete floors really decrease the signal. (I get a "Low" Signal strength report)
I tweaked every setting of my onboard wi-fi notebook adapter (e.g: power management, transmit power) and raised all to get max coverage. Nothing changed.
I even flashed my router with DD-WRT firmware, and pumped the transmit power from 30mW to 251mW. No major change, though...

And now I am left with no other option but to purchase either a Wireless usb adapter for my notebook, either a bigger, better antenna for my Wireless router...
I really am clueless of what to do....
Needing your help here

Thank you,

Alex