MiniPCI cards are a standard form factor (size and connectors) for a variety of devices that can be installed inside a laptop, WiFi cards among them. However, just because you might take the bottom off your laptop and find an unused MiniPCI slot sitting there, doesn't mean that a MiniPCI WiFi card will work. For internal WiFi, there must be an existing antenna pre-installed in the case at the time of manufacture and there must be appropriate switching available in the case/motherboard configuration (either logical, such as toggling via a specific function key, or a physical switch somewhere on the case). These are not things that can be added later by the end user.
It is possible that your model was designed to be sold with a WiFi option, but you just didn't buy that option. Such models are built with the antenna installed (it's just two tiny wires and costs next to nothing to put in during assembly) but the MiniPCI card left out. In such a situation, you could later add an appropriate MiniPCI card yourself. If so, the Dell TrueMobile card is a fine card. Call Dell and see if they can tell you if your model was originally WiFi capable.
If your laptop doesn't have the antenna wires already inside, your only option is one of the many PCCard plugin modules that slips into the slot on the side of your laptop. They have everything needed inside the card - just plug and play.
The a/b/g stuff is about radio frequencies and connection protocols. Kinda like AM and FM. You can't listen to FM stations with an AM radio. But an AM/FM radio can pick up either type station.
WiFi "b" and "g" use the same frequencies, but a different protocol for the connection. "b" is slower while "g" is faster; but "b" doesn't receive "g", while "g" can receive either "b" or "g". Thus, a "g" card is preferable, both for performance when a "g" transmitter is available and for compatibility if only a "b" transmitter is available. The "a" standard is as fast as "g", but is compatible only with an "a" transmitter. For better or worse, the "a" vs. "b/g" battle is much like the old Beta vs. VHS video issue. "a" is like Beta - maybe it was better, but VHS and "b/g" WiFi are more popular. You just don't see "a" used much in WiFi anymore.
I have a Dell Latitude C400 notebook and I wanted to put an internal wireless card in it. It has a 1.2 intel with 512 RAM. What internal cards will work with it? Will any work with it? Does anyone have experience with this model and internal wireless? Also, what do the letters behind wireless cards mean- a/b/g...????