personally I dont think Alfa is the best decision generally because is big but injection is working.
you can use any antenna you want even 100dBi if you find it somewhere.
im not specialist but I did lot of test for last 3 months with many antennas many adapters boosters and Backtrack 4 and 3
most ease to setup and use was zyxell g-202
if you want something small and good range for small money belkin f5d9050b is G+mimo
if you want use it from home and you want good rande outside use your alfa buy 15 or 24 dBi externall antenna with cable 100mm connect antenna to adampter and use example 5m usb cable must be usb repeater if you use long antenna cable you will lose signall strengt what is not happend with usb cable. you can even use amplifire but probobly you will need modificate your driver.
generally is you use comp outside of your homw use something small with good range example belkin
if you use it inside your home use your alfa with externall antenna for good range
next time you buy adapter dont look for adapter that is 500mW more important is if it is G G+ N or N1 also if is working with backtrack