I,d recommend getting a ALFA AWUSO36H USB wireless aadaptor , it transmits at 500mw which is high powered compared to other wireless adaptors and has an external antenna socket,
get a tin can and make a cantenna, plenty of instructions out there, and they are a very good and cheap aerial for wireless..
I made one using a old whisky tin, simple to make and 12db, i attached mine to a small satelite dish too for that extra gain, coupled with the 500mw wireless adaptor i get sme pretty impressive results..
The adaptors also compatable for packet injection too when using airreplay on lnux if you need to use that for any Wep netowrks
this guys got the right idea. just popping an antenna on a normal wifi card is like putting a heavier boot on an accelerator pedal............you always end up in the same place just a bit faster lol.
the alfa networks usb card is the dogs danglys m8, 500mw of transmit power on a receive sensi of about 93dBi is just waht you`ll need. and its full compatible with backtrack (all ver), by the way "wesside-ng -i wlan0 -v `victims mac` " is the best command on bt3 lol.
you can pick them up for £43 on e-bay now, just bought another yesterday. bargain and well worth the money.
lots of things to take into account when it comes to wireless though, dBi / dBm signal loss etc etc.
your best bet would be this.
alfa networks usb card.
mini b female to mini male usb extention lead (hard to get hold of, may have to make one)
decent 12dbi omni antenna.
cheape e-bay mag base .
this is how ive got one set up.
ive fixed a small metal plate to the side of my house on the second floor to which is fixed my el cheapo mag base and a cheapo 12dBi omni antenna.
the patch lead (rpsma) comes through the wall, its a very short lead only 1 meter in lengh so only a couple of dBi signal loss, if you run a long cable from the antenna to the client / access point you will loose alot of signal , hence why new antennas have built in access points or clients so the leads can be short, no signal is lost over digital. so from my mag mount to client (alfa) there is the pigtail 1m cable, and then from the usb alfa networks client i have a custom made 7 meter usb lead, mini b female to mini b male. this then connects to the standard dual usb alfa cable and goes into my pc.
on this little setup i get many, many networks. i can even connect to a school 2 miles away in direct line of site
lot of factors to consider, but seriously , get the alfa, use backtrack to crack networks and play (your own of course
) save spending money on equipment which isnt up to the job.
hope this helps.