I'd recommend an Alfa AWUS036H with at least a 7dBi panel antenna. It's cheap and high powered. If you're going to use omni-directional antennas like what your current WiFi adapter is using, be sure to orient them correctly. Imagine the radiation pattern of an omni-directional antenna looking like a doughnut when it is pointing straight upwards - the frequencies radiate out of the sides better than through the top, so you don't want to aim the antennas as if you were pointing with your index finger. Another tip I have would be to buy a better WiFi router with external antennas, preferably one capable of running either TomatoUSB, DD-WRT, or OpenWRT router firmwares. If you want to go really long distances, you can buy a parabolic grid antenna, but that isn't practical for indoors. Yagi WiFi antennas are good by design, but all of them being sold aren't correctly manufactured - the elements aren't spaced correctly, aren't the correct lengths, and aren't made of decent materials. If you want a good yagi antenna, you'd pretty much have to build it yourself.