Wireless cards depend on a couple of things:
Notebook or Laptop
What type of environment do you live in? (Apartment complex/multi family home, private home)
Which band you want/need to use
How "bleeding edge" you want to go
If you have a notebook and your computer has a MINI-PCI slot, then your best bet is to get a minipci network card. Chances are your laptop is already wired for an antanna through the LCD case and you don't need a stupid dicfer hanging out. If it doesn't you now have 2 choices: You can get a USB thumbcard, or a PCMCIA card.
Typically, you'll get better reception from a PCMCIA card over longer distances, but they tend to be more difficult to leave in your laptop when you carry it in a case. Most top loading bags will support your USB dicfer if your USB slots are in the back.
Price wise, the USB cards will run you from 45-75 for a 802.11A/G card while the PCMCIA will run you roughly 30-60 for the same. Do NOT invest in a setup that supports only 802.11B as thats kind of outdated and slow at this point if you have other computers in your home and you plan on sharing files between machines.
A minipci based card will run you anywhere from $20 to $90 depending on whether you use eBay.
On your desktop clunkers, you have some more choices to make:
Some cards come with an extension wire so you can move your actual antanna away from the 'puter by about 5-10 feet. I STRONLY reccomend this as it can help you avoid interference from the power supply and CPU. These cards tend to run about $20 more than your standard antanna in the back of the card variety. Good cards start at around $50 and can run as high as $100 for top of the line cards.
One last comment: There'* a band called "Pre-N". I strongly reccomend you avoid it like the bubonic plague. No standards have been agreed on there and you could find yourself with nonbackwards compatibile crap that could be interfered with by other devices.
1995 SSEi - Totaled on 6/6/6 by some jerk who drank at 9am. WEAR YOUR SEATBELT! It saved my life and it can save yours too.