finding longest range wifi adapter

If you get a Wireless N adapter then your range will increase greatly
802.11g
The 802.11g protocol was introduced in 2003, by which time consumers were suffering a bit from the slow data rates of 802.11b. With an increased data rate of 54 Mbit/s and even more saving, due to aggressive pricing and saving in production cost, 802.11g became the protocol of choice. This protocol worked in the same 2.4 frequency band of the 802.11b protocol. In average conditions, 802.11b had an indoor range of about 38 meters and an outdoor range of about 140 meters, the same as the 802.1b protocol. However, due to the increased data rate, decreased cost, and backward compatibility with 802.11b and 802.11a, this was, and still is, the most popular Wi-Fi protocol used. Unfortunately, many other devices, such as Bluetooth technology, cordless phones, microwaves, etc. also used the same 2.4 GHz band, and thus there was a lot of interference in this protocol. To counter this, the IEEE came up with the 802.11n protocol.

802.11n
Introduced in 2009, the main difference in 802.11n was the presence of 3 more MIMO streams than the other protocols. While all the other protocols used just a single MIMO stream, the 802.11n protocol used 4 MIMO streams. This not only helped in an increase in the range, but also an increase in the data rate. Using 4 antennas as compared to the usual 1 antenna in other protocols, 802.11n can achieve a data rate of 300 Mbit/s. In average conditions, 802.11n has an indoor range of about 70 meters and an outdoor range of about 250 meters. Wireless n also could operate on both, 2.4 GHz and the 5 GHz bands. Using the 5 GHz band eliminated any interference from other devices using the 2.4GHz band and improved connectivity.

it may be big, but it is awesome. The HornetTek Janus WLAN Adapter has an incredible receiving range.

You could also apply a WiFi range extender that you install half way between the router and the PC. Example: Newegg.com - NETGEAR WN2000RPT-100NAS 802.11b/g/n Universal WiFi Range Extender

Look on ebay

There are either 'yagi' directionals (13-20db outside booster aerials)

Ebay uk get yerself a couple of these highly recommended - if you have twin aerials on 'n' wifi card (for mimo reception) or directional router boost (might be illegal in your country) for garden use etc

WiFi 8.5dBi Extender Booster-Wireless Panel Antenna | eBay UK

Swiss made and do what they say on the tin!

(my two pick up signals from an internet cafe half a mile away in the opposite direction to where these point to the router)

They are actually used to catch people in large department stores who nick stuff 'cos they sense the hidden foil markers - work really well with wifi cards though)
Don't buy the black booster 'stick' antennae/aerials - they're rubbish!