The main thing is to figure out what kind of expansion slot you have, if any, and find the correct wireless card for that slot. Many laptops don't have any expansion slot at all, in which case your only option for WiFi would be a USB dongle. However, laptops that do have an expansion slot almost always use PC Card slots (formerly PCMCIA), in which case you would just need to find a PC Card WiFi expansion card that is compatible with whatever operating system you are running.
If your laptop is a Mac, you hopefully have an AirPort or AirPort Extreme slot, in which case you'll need the appropriate card from Apple, but otherwise I seriously doubt you would have any expansion slot other than PC Card.
Once you get it working, the hard disk and RAM requirements for the driver are usually trivial compared to the cost of running internet browsers and other large programs. It is likely to hurt you in battery life more than anything else. You can get online any place that has a WiFi hotspot (some libraries have free ones, some StarBucks have paid ones, and some people build their own in their house with a WiFi router).