i don't think your problem is the signal strength, it's that all the other routers are so close that their signals are as strong as yours, swamping it.
a directional antenna would help, not by increasing your routers received strength, but by it's directionality allowing you to block out the others.
i use a usb wireless network adapter on a usb extension cable at the focus of a parabolic reflector i made out of a thin piece of screening which i then covered in tin foil. i used a section of packing foam cut to shape to maintain the parabola. (it's only bent in one direction, it's not a dish). anyhow, with it i get a signal in the 80% range, without it is down to about 10% as i am about 50 ft. from the router location with a few walls in between.
a simpler one would be to make a small corner reflector.
see this linky. the reflector is put on the router antenna in the linked article, but could also be put on the pc's antenna. rotate till the offending routers are blocked while your signal is best. if that helps, you could make one of the more complex directionals.
hopefully your pc's adapter is also an N+, a std. 802.11G pc adapter would work, but not as effectively. try seeing if the adapter's advanced settings have any parameters you can tweak as i mentioned earlier.