|Does your existing router support "bridge mode"?|
If yes, you can get another router that also does "bridge mode" and then you create a wireless bridge between the two to extend the range of your wireless network.
Alternatively, you could connect a second wireless capable router to your first one with a network cable.
Tomato, while a great 3'd party firmware can't actually boost the power of the router's wireless network. Think about it for a minute and ask yourself how a piece of software could make any piece of hardware exceed it's power rating and/or capacity. Could upgrading the software in your car's onboard computer suddenly jump it's horsepower from 100 to 200 horses? I don't think so.......
I would also google "wireless range extender" and see what you find. In fact, now that I think about it, I'd start there.