Some observations from personal experience.
I'm not sure this will help, but I have used a laptop for the last three years that had built a in Wi-Fi card. In most cases, I found the reception to be inadequate, especially while in my Airstream. Consequently, I purchased a Wi-Fi card with an external antenna connection (MC card) to fit in the PCMCIA slot. This card, along with an external antenna, allowed me to get outside the aluminum, and significantly improved reception. This type of card is plentiful, and cost between $40 - $80. I also used this PCMCIA slot for my Verizon 595 broadband capable aircard, which also worked very nicely.
I just bought a new laptop with built in Wi-Fi and was faced with a choice of getting one with the "new" standard interface, which is the Express Card, or finding one with the PCMCIA slot so I can use my existing WI-Fi card. During my quest, I had noticed that there aren't too many peripheral devices made for this new Express Card slot. For instance, Verizon only has one aircard at this time that will fit the Express Card slot. This observation was confirmed by sales reps. from Verizon, Dell, and Gateway. I ended up getting a laptop with a PCMCIA slot so I could still use my existing Wi-Fi card, and upgraded to a USB broadband capable aircard. Again, it had an external antenna connection to get outside the Airstream. With the slot standards (PCMCIA/Express Card) still in transition, the safest bet may be to go with USB. There are numerous Wi-Fi adapters, some with external antenna connectors, made for USB ports in the $50-$90 range.
1968 Trade Wind
2007 Ford F250 4x4 Crew