There are a few external 802.11n USB devices that have a removable antenna that you could use with a larger external antenna if the distance to the AP is a little to far.
A better option is to setup a second 802.11n AirPort router as an extender AP to your other AP.
If you are thinking of replacing the internal Airport card to a newer version the jury is still out if the new 802.11ac board will work or not in the older Macs. iFixIt just did a teardown on the new 13" Air and noted the card is a different size and the antenna connections are located in a different location so the cables may not reach.
In truth most people don't really need that much speed. Yes, it's nice to have but at what cost?