I agree with john-b691, "What adapter you get is partially dependent on your router."
if you have a slow Wi-Fi connection, its usually not the adapter, but the router. I had a fast Ethernet connection, but my WiFi was really crappy: my router was a "smart" router that kept limiting the maximum bandwidth to around 125Kb/S, i had to mess around with it for a while to get it to not limit my bandwidth. now i get 175-250 Kb/s.
make sure you know what your wifi router's capabilites are: what the frequency is and if its 802.11 n/b/g , and get an adapter that supports your router.