lol... for wifi 500mw is alot of power, no wayyou'll fry your brain or boil water.
i dont reckon you'd be able to mount somethin outside would ya? you could make a nice little setup to run power over ethernet outside to a little panel antenna, with a radio mounted inside of it. then you run cat5 inside to the POE power injector and then to the PC (or a router or whatever)
if it has to stay inside, you could get a floor standing lamp and mount an antenna on it. you can get a nice little 14 dbdish that dont look to bad, or possibly a YAGI variety, im not all that fond of them tho, i like parabolic dishes.
im talkin about all retail here, which will unfortunately get a wee pricey, sub $200 but able to easily make a LOS link a few miles, pretty much like we use when installing a customer to our wireless. i dunno what to reccomend really for DIY except like freeantennas.com or you can find your own DIY.