Whether or not you want an external antenna depends on your situation. If your AP is 2 feet away from your computer, or you get fine signal strength anyways, then there's no need, and you could even use the built-in wifi of your notebook.
However, if your signal is dropping out at long-range, or if you want to do a little, ahem, "reconaissance" of your neighbors' networks, then you'll want an external antenna. Most PCMCIA wifi cards don't even come with connectors for an external antenna, but you might like a good all-around card such as the Proxim 8470-WD. It comes with an MC-style connector, and it's Atheros-based, so it works well with Linux.
If money is no object, and you want to most range possible, look at the Ubiquiti SRC 300mW a/b/g card (
http://www.ubnt.com/super_range_cardbus.php4). It's got smoking transmit power, works with 802.11/a/b/g, and has *two* MMCX antenna connectors. Just beware that this card, unlike the Proxim, does NOT have an internal antenna so you'll have to use at least a tiny external antenna at all times.
I hoped this helped somewhat!