Although im going through walls, not line of sight, the solution I came upon was to use a 4 port usb extender hub by CAT5:http://www.amazon.co.uk/gp/product/B001R022ZI/ref=oss_product
id put one hub by the computer and the other near to where I would sit in the living room. I can then use a cheep short range wireless keyboard and mouse when i get around to buying one, or stay wired.
I would also recommend looking into keyboards that claim to have a good range, 2.4ghz keyboards perhaps..http://www.ceratech.co.uk/cpages/keyboards/rf/kyb-toughball.htm
UK solution, but im sure there are some in the US.
As in my earlier post, a RF wireless keyboard with the range that you want is going to cots $200+!
unfortunatly the market is saturated in wireless desktop range (1 - 3 m) keyboard and mouse, and not htpc range (3m+) desktop style wireless keyboard and mouse.